High throughput nucleic acid sequencing by expansion

ABSTRACT

Nucleic acid sequencing methods and related products are disclosed. Methods for sequencing a target nucleic acid comprise providing a daughter strand produced by a template-directed synthesis, the daughter strand comprising a plurality of subunits coupled in a sequence corresponding to a contiguous nucleotide sequence of all or a portion of the target nucleic acid, wherein the individual subunits comprise a tether, at least one probe or nucleobase residue, and at least one selectively cleavable bond. The selectively cleavable bond(s) is/are cleaved to yield an Xpandomer of a length longer than the plurality of the subunits of the daughter strand, the Xpandomer comprising the tethers and reporter elements for parsing genetic information in a sequence corresponding to the contiguous nucleotide sequence of all or a portion of the target nucleic acid. Reporter elements of the Xpandomer are then detected. Corresponding products, including Xpandomers and oligomeric and monomeric substrate constructs are also disclosed.

CROSS-REFERENCES TO RELATED APPLICATIONS

The present application claims the benefit under 35 U.S.C. §119(e) ofU.S. Provisional Patent Application No. 60/945,031 filed on Jun. 19,2007; U.S. Provisional Patent Application No. 60/981,916 filed on Oct.23, 2007; and U.S. Provisional Patent Application No. 61/000,305 filedon Oct. 25, 2007; all of which are incorporated herein by reference intheir entireties.

BACKGROUND

1. Technical Field

This invention is generally related to nucleic acid sequencing, as wellas methods and products relating to the same.

2. Description of the Related Art

Nucleic acid sequences encode the necessary information for livingthings to function and reproduce, and are essentially a blueprint forlife. Determining such sequences is therefore a tool useful in pureresearch into how and where organisms live, as well as in appliedsciences such drug development. In medicine, sequencing tools can beused for diagnosis and to develop treatments for a variety ofpathologies, including cancer, heart disease, autoimmune disorders,multiple sclerosis, or obesity. In industry, sequencing can be used todesign improved enzymatic processes or synthetic organisms. In biology,such tools can be used to study the health of ecosystems, for example,and thus have a broad range of utility.

An individual's unique DNA sequence provides valuable informationconcerning their susceptibility to certain diseases. The sequence willprovide patients with the opportunity to screen for early detection andto receive preventative treatment. Furthermore, given a patient'sindividual blueprint, clinicians will be capable of administeringpersonalized therapy to maximize drug efficacy and to minimize the riskof an adverse drug response. Similarly, determining the blueprint ofpathogenic organisms can lead to new treatments for infectious diseasesand more robust pathogen surveillance. Whole genome DNA sequencing willprovide the foundation for modern medicine.

DNA sequencing is the process of determining the order of the chemicalconstituents of a given DNA polymer. These chemical constituents, whichare called nucleotides, exist in DNA in four common forms:deoxyadenosine (A), deoxyguanosine (G), deoxycytidine (C), anddeoxythymidine (T). Sequencing of a diploid human genome requiresdetermining the sequential order of approximately 6 billion nucleotides.

Currently, most DNA sequencing is performed using the chain terminationmethod developed by Frederick Sanger. This technique, termed SangerSequencing, uses sequence specific termination of DNA synthesis andfluorescently modified nucleotide reporter substrates to derive sequenceinformation. This method sequences a target nucleic acid strand, or readlength, of up to 1000 bases long by using a modified polymerase chainreaction. In this modified reaction the sequencing is randomlyinterrupted at select base types (A, C, G or T) and the lengths of theinterrupted sequences are determined by capillary gel electrophoresis.The length then determines what base type is located at that length.Many overlapping read lengths are produced and their sequences areoverlaid using data processing to determine the most reliable fit of thedata. This process of producing read lengths of sequence is verylaborious and expensive and is now being superseded by new methods thathave higher efficiency.

The Sanger method was used to provide most of the sequence data in theHumane Genome Project which generated the first complete sequence of thehuman genome. This project took over 10 years and nearly $3 B tocomplete. Given these significant throughput and cost limitations, it isclear that DNA sequencing technologies will need to improve drasticallyin order to achieve the stated goals put forth by the scientificcommunity. To that end, a number of second generation technologies,which far exceed the throughput and cost per base limitations of Sangersequencing, are gaining an increasing share of the sequencing market.Still, these “sequencing by synthesis” methods fall short of achievingthe throughput, cost, and quality targets required by markets such aswhole genome sequencing for personalized medicine.

For example, 454 Life Sciences is producing instruments (e.g., theGenome Sequencer) that can process 100 million bases in 7.5 hours withan average read length of 200 nucleotides. Their approach uses avariation of Polymerase Chain Reaction (“PCR”) to produce a homogeneouscolony of target nucleic acid, hundreds of bases in length, on thesurface of a bead. This process is termed emulsion PCR. Hundreds ofthousands of such beads are then arranged on a “picotiter plate”. Theplate is then prepared for an additional sequencing whereby each nucleicacid base type is sequentially washed over the plate. Beads with targetthat incorporate the base produce a pyrophosphate byproduct that can beused to catalyze a light producing reaction that is then detected with acamera.

Illumina Inc. has a similar process that uses reversibly terminatingnucleotides and fluorescent labels to perform nucleic acid sequencing.The average read length for Illumina's 1G Analyzer is less than 40nucleotides. Instead of using emulsion PCR to amplify sequence targets,Illumina has an approach for amplifying PCR colonies on an arraysurface. Both the 454 and Illumina approaches use a complicatingpolymerase amplification to increase signal strength, perform basemeasurements during the rate limiting sequence extension cycle, and havelimited read lengths because of incorporation errors that degrade themeasurement signal to noise proportionally to the read length.

Applied Biosystems uses reversible terminating ligation rather thansequencing-by-synthesis to read the DNA. Like 454's Genome Sequencer,the technology uses bead-based emulsion PCR to amplify the sample. Sincethe majority of the beads do not carry PCR products, the researchersnext use an enrichment step to select beads coated with DNA. Thebiotin-coated beads are spread and immobilized on a glass slide arraycovered with streptavidin. The immobilized beads are then run through aprocess of 8-mer probe hybridization (each labeled with four differentfluorescent dyes), ligation, and cleavage (between the 5th and 6th basesto create a site for the next round of ligation). Each probeinterrogates two bases, at positions 4 and 5 using a 2-base encodingsystem, which is recorded by a camera. Similar to Illumina's approach,the average read length for Applied Biosystems' SOLiD platform is lessthan 40 nucleotides.

Other approaches are being developed to avoid the time and expense ofthe polymerase amplification step by measuring single molecules of DNAdirectly. Visigen Biotechnologies, Inc. is measuring fluorescentlylabeled bases as they are sequenced by incorporating a secondfluorophore into an engineered DNA polymerase and using ForsterResonance Energy Transfer (FRET) for nucleotide identification. Thistechnique is faced with the challenges of separating the signals ofbases that are separated by less than a nanometer and by a polymeraseincorporation action that will have very large statistical variation.

A process being developed by LingVitae sequences cDNA inserted intoimmobilized plasmid vectors. The process uses a Class IIS restrictionenzyme to cleave the target nucleic acid and ligate an oligomer into thetarget. Typically, one or two nucleotides in the terminal 5′ or 3′overhang generated by the restriction enzyme determine which of alibrary of oligomers in the ligation mix will be added to the sticky,cut end of the target. Each oligomer contains “signal” sequences thatuniquely identify the nucleotide(s) it replaces. The process of cleavageand ligation is then repeated. The new molecule is then sequenced usingtags specific for the various oligomers. The product of this process istermed a “Design Polymer” and always consists of a nucleic acid longerthan the one it replaces (e.g., a dinucleotide target sequence isreplaced by a “magnified” polynucleotide sequence of as many as 100 basepairs). An advantage of this process is that the duplex product strandcan be amplified if desired. A disadvantage is that the process isnecessarily cyclical and the continuity of the template would be lost ifsimultaneous multiple restriction cuts were made.

U.S. Pat. No. 7,060,440 to Kless describes a sequencing process thatinvolves incorporating oligomers by polymerization with a polymerase. Amodification of the Sanger method, with end-terminated oligomers assubstrates, is used to build sequencing ladders by gel electrophoresisor capillary chromatography. While coupling of oligomers by end ligationis well known, the use of a polymerase to couple oligomers in atemplate-directed process was utilized to new advantage.

Polymerization techniques are expected to grow in power as modifiedpolymerases (and ligases) become available through genetic engineeringand bioprospecting, and methods for elimination of exonuclease activityby polymerase modification are already known. For example, PublishedU.S. Patent Application 2007/0048748 to Williams describes the use ofmutant polymerases for incorporating dye-labeled and other modifiednucleotides. Substrates for these polymerases also include γ-phosphatelabeled nucleotides. Both increased speed of incorporation and reductionin error rate were found with chimeric and mutant polymerases.

In addition, a large effort has been made by both academic andindustrial teams to sequence native DNA using non-synthetic methods. Forexample, Agilent Technologies, Inc. along with university collaboratorsare developing a single molecule detection method that threads the DNAthrough a nanopore to make measurements as it passes through. As withVisigen and LingVitae, this method must overcome the problem ofefficiently and accurately obtaining distinct signals from individualnucleobases separated by sub-nanometer dimensions, as well as theproblem of developing reproducible pore sizes of similar size. As such,direct sequencing of DNA by detection of its constituent parts has yetto be achieved in a high-throughput process due to the small size of thenucleotides in the chain (about 4 Angstroms center-to-center) and thecorresponding signal to noise and signal resolution limitations therein.Direct detection is further complicated by the inherent secondarystructure of DNA, which does not easily elongate into a perfectly linearpolymer.

While significant advances have been made in the field of DNAsequencing, there continues to be a need in the art for new and improvedmethods. The present invention fulfills these needs and provides furtherrelated advantages.

BRIEF SUMMARY

In general terms, methods and corresponding devices and products aredisclosed that overcome the spatial resolution challenges presented byexisting high throughput nucleic acid sequencing techniques. This isachieved by encoding the nucleic acid information on a surrogate polymerof extended length which is easier to detect. The surrogate polymer(referred to herein as an “Xpandomer”) is formed by template directedsynthesis which preserves the original genetic information of the targetnucleic acid, while also increasing linear separation of the individualelements of the sequence data.

In one embodiment, a method is disclosed for sequencing a target nucleicacid, comprising: a) providing a daughter strand produced by atemplate-directed synthesis, the daughter strand comprising a pluralityof subunits coupled in a sequence corresponding to a contiguousnucleotide sequence of all or a portion of the target nucleic acid,wherein the individual subunits comprise a tether, at least one probe ornucleobase residue, and at least one selectively cleavable bond; b)cleaving the at least one selectively cleavable bond to yield anXpandomer of a length longer than the plurality of the subunits of thedaughter strand, the Xpandomer comprising the tethers and reporterelements for parsing genetic information in a sequence corresponding tothe contiguous nucleotide sequence of all or a portion of the targetnucleic acid; and c) detecting the reporter elements of the Xpandomer.

In more specific embodiments, the reporter elements for parsing thegenetic information may be associated with the tethers of the Xpandomer,with the daughter strand prior to cleavage of the at least oneselectively cleavable bond, and/or with the Xpandomer after cleavage ofthe at least one selectively cleavable bond. The Xpandomer may furthercomprise all or a portion of the at least one probe or nucleobaseresidue, and the reporter elements for parsing the genetic informationmay be associated with the at least one probe or nucleobase residue ormay be the probe or nucleobase residues themselves. Further, theselectively cleavable bond may be a covalent bond, an intra-tether bond,a bond between or within probes or nucleobase residues of the daughterstrand, and/or a bond between the probes or nucleobase residues of thedaughter strand and a target template.

In further embodiments, Xpandomers have the following structures (I)through (X):

Structure (I):

wherein

-   -   T represents the tether;    -   P¹ represents a first probe moiety;    -   P² represents a second probe moiety;    -   κ represents the κ^(th) subunit in a chain of m subunits, where        m is an integer greater than three; and    -   α represents a species of a subunit motif selected from a        library of subunit motifs, wherein each of the species comprises        sequence information of the contiguous nucleotide sequence of a        portion of the target nucleic acid.

Structure (II):

wherein

-   -   T represents the tether;    -   P¹ represents a first probe moiety;    -   P² represents a second probe moiety;    -   κ represents the κ^(th) subunit in a chain of m subunits, where        m is an integer greater than three;    -   α represents a species of a subunit motif selected from a        library of subunit motifs, wherein each of the species comprises        sequence information of the contiguous nucleotide sequence of a        portion of the target nucleic acid; and    -   χ represents a bond with the tether of an adjacent subunit.

Structure (III):

wherein

-   -   T represents the tether;    -   P¹ represents a first probe moiety;    -   P² represents a second probe moiety;    -   κ represents the κ^(th) subunit in a chain of m subunits, where        m is an integer greater than three;    -   α represents a species of a subunit motif selected from a        library of subunit motifs, wherein each of the species comprises        sequence information of the contiguous nucleotide sequence of a        portion of the target nucleic acid; and    -   χ represents a bond with the tether of an adjacent subunit.

Structure (IV):

wherein

-   -   T represents the tether;    -   P¹ represents a first probe moiety;    -   P² represents a second probe moiety;    -   κ represents the κ^(th) subunit in a chain of m subunits, where        m is an integer greater than three;    -   α represents a species of a subunit motif selected from a        library of subunit motifs, wherein each of the species comprises        sequence information of the contiguous nucleotide sequence of a        portion of the target nucleic acid; and    -   χ represents a bond with the tether of an adjacent subunit.

Structure (V):

wherein

-   -   T represents the tether;    -   κ represents the κ^(th) subunit in a chain of m subunits, where        m is an integer greater than three;    -   α represents a species of a subunit motif selected from a        library of subunit motifs, wherein each of the species comprises        sequence information of the contiguous nucleotide sequence of a        portion of the target nucleic acid; and    -   χ represents a bond with the tether of an adjacent subunit.

Structure (VI):

wherein

-   -   T represents the tether;    -   N represents a nucleobase residue;    -   κ represents the κ^(th) subunit in a chain of m subunits, where        m is an integer greater than ten;    -   α represents a species of a subunit motif selected from a        library of subunit motifs, wherein each of the species comprises        sequence information of the contiguous nucleotide sequence of a        portion of the target nucleic acid; and    -   χ represents a bond with the tether of an adjacent subunit.

Structure (VII):

wherein

-   -   T represents the tether;    -   κ represents the κ^(th) subunit in a chain of m subunits, where        m is an integer greater than ten;    -   α represents a species of a subunit motif selected from a        library of subunit motifs, wherein each of the species comprises        sequence information of the contiguous nucleotide sequence of a        portion of the target nucleic acid; and    -   χ represents a bond with the tether of an adjacent subunit.

Structure (VIII):

wherein

-   -   T represents the tether;    -   N represents a nucleobase residue;    -   κ represents the κ^(th) subunit in a chain of m subunits, where        m is an integer greater than ten;    -   α represents a species of a subunit motif selected from a        library of subunit motifs, wherein each of the species comprises        sequence information of the contiguous nucleotide sequence of a        portion of the target nucleic acid; and    -   χ represents a bond with the tether of an adjacent subunit.

Structure (IX):

wherein

-   -   T represents the tether;    -   N represents a nucleobase residue;    -   κ represents the κ^(th) subunit in a chain of m subunits, where        m is an integer greater than ten;    -   α represents a species of a subunit motif selected from a        library of subunit motifs, wherein each of the species comprises        sequence information of the contiguous nucleotide sequence of a        portion of the target nucleic acid;    -   χ¹ represents a bond with the tether of an adjacent subunit; and    -   χ² represents an inter-tether bond.

Structure (X):

wherein

-   -   T represents the tether;    -   n¹ and n² represents a first portion and a second portion,        respectively, of a nucleobase residue;    -   κ represents the κ^(th) subunit in a chain of m subunits, where        m is an integer greater than ten; and

α represents a species of a subunit motif selected from a library ofsubunit motifs, wherein each of the species comprises sequenceinformation of the contiguous nucleotide sequence of a portion of thetarget nucleic acid.

In further embodiments, oligomer substrate constructs for use in atemplate directed synthesis for sequencing a target nucleic acid aredisclosed. Oligomer substrate constructs comprise a first probe moietyjoined to a second probe moiety, each of the first and second probemoieties having an end group suitable for the template directedsynthesis, and a tether having a first end and a second end with atleast the first end of the tether joined to at least one of the firstand second probe moieties, wherein the oligomer substrate construct whenused in the template directed synthesis is capable of forming a daughterstrand comprising a constrained Xpandomer and having a plurality ofsubunits coupled in a sequence corresponding to the contiguousnucleotide sequence of all or a portion of the target nucleic acid,wherein the individual subunits comprise a tether, the first and secondprobe moieties and at least one selectively cleavable bond.

In another embodiment, monomer substrate constructs for use in atemplate directed synthesis for sequencing a target nucleic acid aredisclosed. Monomer substrate constructs comprise a nucleobase residuewith end groups suitable for the template directed synthesis, and atether having a first end and a second end with at least the first endof the tether joined to the nucleobase residue, wherein the monomersubstrate construct when used in the template directed synthesis iscapable of forming a daughter strand comprising a constrained Xpandomerand having a plurality of subunits coupled in a sequence correspondingto the contiguous nucleotide sequence of all or a portion of the targetnucleic acid, wherein the individual subunits comprise a tether, thenucleobase residue and at least one selectively cleavable bond.

In yet further embodiments, template-daughter strand duplexes aredisclosed comprising a daughter strand duplexed with a template strand,as well as to methods for forming the same from the template strand andthe oligomer or monomer substrate constructs.

These and other aspects of the invention will be apparent upon referenceto the attached drawings and following detailed description. To thisend, various references are set forth herein which describe in moredetail certain procedures, compounds and/or compositions, and are herebyincorporated by reference in their entirety.

BRIEF DESCRIPTION OF THE DRAWINGS

In the figures, identical reference numbers identify similar elements.The sizes and relative positions of elements in the figures are notnecessarily drawn to scale and some of these elements are arbitrarilyenlarged and positioned to improve figure legibility. Further, theparticular shapes of the elements as drawn are not intended to conveyany information regarding the actual shape of the particular elements,and have been solely selected for ease of recognition in the figures.

FIGS. 1A and 1B illustrate the limited separation between nucleobasesthat must be resolved in order to determine the sequence of nucleotidesin a nucleic acid target.

FIGS. 2A through 2D illustrate schematically several representativestructures of substrates useful in the invention.

FIGS. 3A, 3B and 3C are schematics illustrating simplified steps forsynthesizing an Xpandomer from a target nucleic acid.

FIG. 4 is a simple model illustrating a FRET nanopore-type device forsequencing an Xpandomer.

FIG. 5 is a plot with channels for red, green, and blue fluorescenceemissions, and illustrates how analog signals can be decoded intodigital information that corresponds to the genetic sequence informationencoded in an Xpandomer. The accompanying table (FIG. 6) shows how thedata is decoded. By employing three multi-state fluorophores, thesequence of bases can be read with high resolution in digital form froma single molecule in real time as the Xpandomer spools through thenanopore.

FIG. 6 is a look-up table from which the data of FIG. 5 is derived.

FIGS. 7A-E are gels of ligation products.

FIG. 8 is an overview of oligomeric Xpandomers.

FIG. 9 is an overview of monomeric Xpandomers.

FIGS. 10A through 10E depict Class I Xpandomers, intermediates andprecursors in a symbolic and graphical form. These precursors are termedXprobes if monophosphates and Xmers if triphosphates.

FIG. 11 is a condensed schematic of a method for synthesis of anXpandomer by solution ligation using an end-terminated hairpin primerand Class I substrate constructs.

FIG. 12 is a condensed schematic of a method for synthesis of anXpandomer by solution ligation using a double-ended hairpin primer andClass I substrate constructs.

FIG. 13 is a condensed schematic of a method for synthesis of anXpandomer by ligation on an immobilized template, without primers, usingClass I substrate constructs.

FIG. 14 is a condensed schematic of a method for synthesis of anXpandomer by cyclical stepwise ligation using reversibly terminatedClass I substrate constructs on templates annealed to immobilizedprimers.

FIG. 15 is a condensed schematic of a method for synthesis of anXpandomer by promiscuous assembly and chemical coupling using Class Isubstrate constructs without primers.

FIG. 16 is a condensed schematic of a method for synthesis of anXpandomer by solution polymerization using a hairpin primer and Class Itriphosphate substrate constructs.

FIG. 17 is a condensed schematic of a method for synthesis of anXpandomer on an immobilized template using Class I triphosphatesubstrate constructs and a polymerase.

FIGS. 18A through 18E depict a Class II Xpandomer, Xpandomerintermediate, and substrate construct in a symbolic and graphicallanguage.

FIGS. 19A through 19E depict a Class III Xpandomer, Xpandomerintermediate and substrate construct in a symbolic and graphicallanguage.

FIG. 20 is a condensed schematic of a method for synthesis of anXpandomer on an immobilized template using Class II substrate constructscombining hybridization and primer-less chemical coupling.

FIG. 21 is a condensed schematic of a method for synthesis of anXpandomer on an immobilized template using a primer, Class II substrateconstructs and a ligase.

FIGS. 22A through 22E depict a Class IV Xpandomer, Xpandomerintermediate, and substrate construct in a symbolic and graphical form.

FIGS. 23A through 23E depict a Class V Xpandomer, Xpandomerintermediate, and substrate construct in a symbolic and graphical form.

FIG. 24 is a condensed schematic of a method for synthesis of anXpandomer by solution polymerization using an adapter primer and Class Vtriphosphate substrate constructs.

FIG. 25 illustrates structures of deoxyadenosine (A), deoxycytosine (C),deoxyguanosine (G), and deoxythymidine (T).

FIGS. 26A and 26B illustrate nucleotides derivatized with functionalgroups.

FIGS. 27A and 27B illustrate probe members incorporating derivatizednucleobases.

FIGS. 28A through 28D illustrate in more detail Class I-IV substrates ofthe invention, here showing examples of selectively cleavable bondcleavage sites in the probe backbone and indicating loop end linkagesbridging the cleavage sites.

FIGS. 29A through 29D illustrate in more detail Class I-IV substrates ofthe invention, here showing examples of selectively cleavable bondcleavage sites in the probe backbone and indicating loop end linkagesbridging the cleavage sites.

FIG. 30 illustrates one method of assembling a “probe-loop” construct,such as an Xprobe or an Xmer.

FIG. 31 illustrates a method of assembling a Class I substrateconstruct, where the loop contains reporter constructs.

FIGS. 32A through 32C illustrate use of PEG as a polymeric tether.

FIGS. 33A through 33D illustrate poly-lysine as a polymeric tether anddendrimeric constructs derived from poly-lysine scaffolds.

FIGS. 34A through 34C illustrate selected methods of tether loop closureintegrated with reporter construct assembly from segments.

FIGS. 35A and 35B illustrate a method for synthesis of an individualreporter segment by randomized polymerization of precursor blocks.

FIGS. 36A through 361 illustrate reporter constructs.

FIG. 37 is a table showing composition and chemical methods forassembling reporter constructs and their corresponding reporter codes.

FIGS. 38A through 38F are adaptors suitable for use inend-functionalization of target nucleic acids.

FIG. 39 is an adaptor cassette for introducing a terminal ANH functionalgroup on a dsDNA template.

FIG. 40 is a schema for immobilizing and preparing a template forsynthesis of an Xpandomer.

FIGS. 41A through 41E illustrate selected physical stretching methods.

FIGS. 42A through 42C illustrate selected electrostretching methods.

FIGS. 43A through 43D illustrate methods, reagents, and adaptors forstretching in gel matrices.

FIGS. 44A through 44C describes construction and use of “drag tags”.

FIG. 45 describes a promiscuous hybridization/ligation based method forsynthesizing an Xpandomer.

FIGS. 46A and B describe nucleobases used for gap filling.

FIGS. 47A and B describe simulations of gap occurrence.

FIGS. 48A and B describe simulations of gap occurrence.

FIG. 49 illustrates how gaps are filled with 2mers and 3mers.

FIGS. 50A and B describe gap filling simulations using combinations of2mers and 3mers.

FIG. 51 illustrates the use of 2mer and 3mer adjuvants to disruptsecondary structure.

FIG. 52 describes bases useful as adjuvants.

FIG. 53 describes nucleotide substitutions used to reduce secondarystructure.

FIG. 54 describes a nanopore detection model with magnetic beadtransport.

FIG. 55 illustrates a conventional nanopore detection method.

FIG. 56 illustrates a transverse electrode nanopore detection method.

FIG. 57 illustrates a microscopic detection method.

FIG. 58 illustrates detection by electron microscopy.

FIG. 59 illustrates detection using atomic force microscopy.

FIGS. 60A through 60E depict a Class VI Xpandomer, Xpandomerintermediate, and substrate construct in a symbolic and graphicallanguage. These precursors are termed RT-NTPs.

FIG. 61 is a condensed schematic of a method for synthesis of anXpandomer on an immobilized template using reversibly terminated ClassVI triphosphate substrate constructs and a polymerase.

FIGS. 62A through 62E depict a Class VII Xpandomer, Xpandomerintermediate, and substrate construct in a symbolic and graphicallanguage. These precursors are termed RT-NTPs.

FIGS. 63A through 63E depict a Class VIII Xpandomer, Xpandomerintermediate, and substrate construct in a symbolic and graphicallanguage. These precursors are termed RT-NTPs.

FIGS. 64A through 64E depict a Class IX Xpandomer, Xpandomerintermediate, and substrate construct in a symbolic and graphicallanguage. These precursors are termed RT-NTPs.

FIG. 65 is a condensed schematic of a method for synthesis of anXpandomer on an immobilized template using Class IX triphosphatesubstrate constructs and a polymerase.

FIGS. 66A through 66E depict a Class X Xpandomer, Xpandomerintermediate, and substrate construct in a symbolic and graphicallanguage. These precursors are termed XNTPs.

FIG. 67 is a condensed schematic of a method for synthesis of anXpandomer by solution polymerization using a hairpin primer and Class Xtriphosphate substrate constructs.

DETAILED DESCRIPTION

In the following description, certain specific details are set forth inorder to provide a thorough understanding of various embodiments.However, one skilled in the art will understand that the invention maybe practiced without these details. In other instances, well-knownstructures have not been shown or described in detail to avoidunnecessarily obscuring descriptions of the embodiments. Unless thecontext requires otherwise, throughout the specification and claimswhich follow, the word “comprise” and variations thereof, such as,“comprises” and “comprising” are to be construed in an open, inclusivesense, that is, as “including, but not limited to.” Further, headingsprovided herein are for convenience only and do not interpret the scopeor meaning of the claimed invention.

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure or characteristicdescribed in connection with the embodiment is included in at least oneembodiment. Thus, the appearances of the phrases “in one embodiment” or“in an embodiment” in various places throughout this specification arenot necessarily all referring to the same embodiment. Furthermore, theparticular features, structures, or characteristics may be combined inany suitable manner in one or more embodiments. Also, as used in thisspecification and the appended claims, the singular forms “a,” “an,” and“the” include plural referents unless the content clearly dictatesotherwise. It should also be noted that the term “or” is generallyemployed in its sense including “and/or” unless the content clearlydictates otherwise.

DEFINITIONS

As used herein, and unless the context dictates otherwise, the followingterms have the meanings as specified below.

“Nucleobase” is a heterocyclic base such as adenine, guanine, cytosine,thymine, uracil, inosine, xanthine, hypoxanthine, or a heterocyclicderivative, analog, or tautomer thereof. A nucleobase can be naturallyoccurring or synthetic. Non-limiting examples of nucleobases areadenine, guanine, thymine, cytosine, uracil, xanthine, hypoxanthine,8-azapurine, purines substituted at the 8 position with methyl orbromine, 9-oxo-N6-methyladenine, 2-aminoadenine, 7-deazaxanthine,7-deazaguanine, 7-deaza-adenine, N4-ethanocytosine, 2,6-diaminopurine,N6-ethano-2,6-diaminopurine, 5-methylcytosine,5-(C3-C6)-alkynylcytosine, 5-fluorouracil, 5-bromouracil, thiouracil,pseudoisocytosine, 2-hydroxy-5-methyl-4-triazolopyridine, isocytosine,isoguanine, inosine, 7,8-dimethylalloxazine, 6-dihydrothymine,5,6-dihydrouracil, 4-methyl-indole, ethenoadenine and the non-naturallyoccurring nucleobases described in U.S. Pat. Nos. 5,432,272 and6,150,510 and PCT applications WO 92/002258, WO 93/10820, WO 94/22892,and WO 94/24144, and Fasman (“Practical Handbook of Biochemistry andMolecular Biology”, pp. 385-394, 1989, CRC Press, Boca Raton, LO), allherein incorporated by reference in their entireties.

“Nucleobase residue” includes nucleotides, nucleosides, fragmentsthereof, and related molecules having the property of binding to acomplementary nucleotide. Deoxynucleotides and ribonucleotides, andtheir various analogs, are contemplated within the scope of thisdefinition. Nucleobase residues may be members of oligomers and probes.“Nucleobase” and “nucleobase residue” may be used interchangeably hereinand are generally synonymous unless context dictates otherwise.

“Polynucleotides”, also called nucleic acids, are covalently linkedseries of nucleotides in which the 3′ position of the pentose of onenucleotide is joined by a phosphodiester group to the 5′ position of thenext. DNA (deoxyribonucleic acid) and RNA (ribonucleic acid) arebiologically occurring polynucleotides in which the nucleotide residuesare linked in a specific sequence by phosphodiester linkages. As usedherein, the terms “polynucleotide” or “oligonucleotide” encompass anypolymer compound having a linear backbone of nucleotides.Oligonucleotides, also termed oligomers, are generally shorter chainedpolynucleotides.

“Complementary” generally refers to specific nucleotide duplexing toform canonical Watson-Crick base pairs, as is understood by thoseskilled in the art. However, complementary as referred to herein alsoincludes base-pairing of nucleotide analogs, which include, but are notlimited to, 2′-deoxyinosine and 5-nitroindole-2′-deoxyriboside, whichare capable of universal base-pairing with A, T, G or C nucleotides andlocked nucleic acids, which enhance the thermal stability of duplexes.One skilled in the art will recognize that hybridization stringency is adeterminant in the degree of match or mismatch in the duplex formed byhybridization.

“Nucleic acid” is a polynucleotide or an oligonucleotide. A nucleic acidmolecule can be deoxyribonucleic acid (DNA), ribonucleic acid (RNA), ora combination of both. Nucleic acids are generally referred to as“target nucleic acids” or “target sequence” if targeted for sequencing.Nucleic acids can be mixtures or pools of molecules targeted forsequencing.

“Probe” is a short strand of nucleobase residues, referring generally totwo or more contiguous nucleobase residues which are generallysingle-stranded and complementary to a target sequence of a nucleicacid. As embodied in “Substrate Members” and “Substrate Constructs”,probes can be up to 20 nucleobase residues in length. Probes may includemodified nucleobase residues and modified intra-nucleobase bonds in anycombination. Backbones of probes can be linked together by any of anumber of types of covalent bonds, including, but not limited to, ester,phosphodiester, phosphoramide, phosphonate, phosphorothioate,phosphorothiolate, amide bond and any combination thereof. The probe mayalso have 5′ and 3′ end linkages that include, but are not limited to,the following moieties: monophosphate, triphosphate, hydroxyl, hydrogen,ester, ether, glycol, amine, amide, and thioester.

“Selective hybridization” refers to specific complementary binding.Polynucleotides, oligonucleotides, probes, nucleobase residues, andfragments thereof selectively hybridize to target nucleic acid strands,under hybridization and wash conditions that minimize nonspecificbinding. As known in the art, high stringency conditions can be used toachieve selective hybridization conditions favoring a perfect match.Conditions for hybridization such as salt concentration, temperature,detergents, PEG, and GC neutralizing agents such as betaine can bevaried to increase the stringency of hybridization, that is, therequirement for exact matches of C to base pair with G, and A to basepair with T or U, along a contiguous strand of a duplex nucleic acid.

“Template-directed synthesis”, “template-directed assembly”,“template-directed hybridization”, “template-directed binding” and anyother template-directed processes, refer to a process whereby nucleobaseresidues or probes bind selectively to a complementary target nucleicacid, and are incorporated into a nascent daughter strand. A daughterstrand produced by a template-directed synthesis is complementary to thesingle-stranded target from which it is synthesized. It should be notedthat the corresponding sequence of a target strand can be inferred fromthe sequence of its daughter strand, if that is known.“Template-directed polymerization” and “template-directed ligation” arespecial cases of template-directed synthesis whereby the resultingdaughter strand is polymerized or ligated, respectively.

“Contiguous” indicates that a sequence continues without interruption ormissed nucleobase. The contiguous sequence of nucleotides of thetemplate strand is said to be complementary to the contiguous sequenceof the daughter strand.

“Substrates” or “substrate members” are oligomers, probes or nucleobaseresidues that have binding specificity to the target template. Thesubstrates are generally combined with tethers to form substrateconstructs. Substrates of substrate constructs that form the primarybackbone of the daughter strand are also substrates or substrate membersof the daughter strand.

“Substrate constructs” are reagents for template-directed synthesis ofdaughter strands, and are generally provided in the form of libraries.Substrate constructs generally contain a substrate member forcomplementary binding to a target template and either a tether member ortether attachment sites to which a tether may be bonded. Substrateconstructs are provided in a variety of forms adapted to the invention.Substrate constructs include both “oligomeric substrate constructs”(also termed “probe substrate constructs”) and “monomeric substrateconstructs” (also termed “nucleobase substrate constructs”).

“Subunit motif” or “motif” refers to a repeating subunit of a polymerbackbone, the subunit having an overall form characteristic of therepeating subunits, but also having species-specific elements thatencode genetic information. Motifs of complementary nucleobase residuesare represented in libraries of substrate constructs according to thenumber of possible combinations of the basic complementary sequencebinding nucleobase elements in each motif. If the nucleobase bindingelements are four (e.g., A, C, G, and T), the number of possible motifsof combinations of four elements is 4^(x), where x is the number ofnucleobase residues in the motif. However, other motifs based ondegenerate pairing bases, on the substitution of uracil for thymidine inribonucleobase residues or other sets of nucleobase residues, can leadto larger libraries (or smaller libraries) of motif-bearing substrateconstructs. Motifs are also represented by species-specific reporterconstructs, such as the reporters making up a reporter tether. Generallythere is a one-to-one correlation between the reporter construct motifidentifying a particular substrate species and the bindingcomplementarity and specificity of the motif.

“Xpandomer intermediate” is an intermediate product (also referred toherein as a “daughter strand”) assembled from substrate constructs, andis formed by a template-directed assembly of substrate constructs usinga target nucleic acid template. Optionally, other linkages betweenabutted substrate constructs are formed which may include polymerizationor ligation of the substrates, tether-to-tether linkages ortether-to-substrate linkages. The Xpandomer intermediate contains twostructures; namely, the constrained Xpandomer and the primary backbone.The constrained Xpandomer comprises all of the tethers in the daughterstrand but may comprise all, a portion or none of the substrate asrequired by the method. The primary backbone comprises all of theabutted substrates. Under the process step in which the primary backboneis fragmented or dissociated, the constrained Xpandomer is no longerconstrained and is the Xpandomer product which is extended as thetethers are stretched out. “Duplex daughter strand” refers to anXpandomer intermediate that is hybridized or duplexed to the targettemplate.

“Primary backbone” refers to a contiguous or segmented backbone ofsubstrates of the daughter strand. A commonly encountered primarybackbone is the ribosyl 5′-3′ phosphodiester backbone of a nativepolynucleotide. However, the primary backbone of an daughter strand maycontain analogs of nucleobases and analogs of oligomers not linked byphosphodiester bonds or linked by a mixture of phosphodiester bonds andother backbone bonds, which include, but are not limited to followinglinkages: phosphorothioate, phosphorothiolate, phosphonate,phosphoramidate, and peptide nucleic acid “PNA” backbone bonds whichinclude phosphono-PNA, serine-PNA, hydroxyproline-PNA, and combinationsthereof. Where the daughter strand is in its duplex form (i.e., duplexdaughter strand), and substrates are not covalently bonded between thesubunits, the substrates are nevertheless contiguous and form theprimary backbone of the daughter strand.

“Constrained Xpandomer” is an Xpandomer in a configuration before it hasbeen expanded. The constrained Xpandomer comprises all tether members ofthe daughter strand. It is constrained from expanding by at least onebond or linkage per tether attaching to the primary backbone. During theexpansion process, the primary backbone of the daughter strand isfragmented or dissociated to transform the constrained Xpandomer into anXpandomer.

“Constrained Xpandomer backbone” refers to the backbone of theconstrained Xpandomer. It is a synthetic covalent backbone co-assembledalong with the primary backbone in the formation of the daughter strand.In some cases both backbones may not be discrete but may both have thesame substrate or portions of the substrate in their composition. Theconstrained Xpandomer backbone always comprises the tethers whereas theprimary backbone comprises no tether members.

“Xpandomer” or “Xpandomer product” is a synthetic molecular constructproduced by expansion of a constrained Xpandomer, which is itselfsynthesized by template-directed assembly of substrate constructs. TheXpandomer is elongated relative to the target template it was producedfrom. It is composed of a concatenation of subunits, each subunit amotif, each motif a member of a library, comprising sequenceinformation, a tether and optionally, a portion, or all of thesubstrate, all of which are derived from the formative substrateconstruct. The Xpandomer is designed to expand to be longer than thetarget template thereby lowering the linear density of the sequenceinformation of the target template along its length. In addition, theXpandomer optionally provides a platform for increasing the size andabundance of reporters which in turn improves signal to noise fordetection. Lower linear information density and stronger signalsincrease the resolution and reduce sensitivity requirements to detectand decode the sequence of the template strand.

“Selectively cleavable bond” refers to a bond which can be broken undercontrolled conditions such as, for example, conditions for selectivecleavage of a phosphorothiolate bond, a photocleavable bond, aphosphoramide bond, a 3′-O—B-D-ribofuranosyl-2′ bond, a thioether bond,a selenoether bond, a sulfoxide bond, a disulfide bond,deoxyribosyl-5′-3′ phosphodiester bond, or a ribosyl-5′-3′phosphodiester bond, as well as other cleavable bonds known in the art.A selectively cleavable bond can be an intra-tether bond or between orwithin a probe or a nucleobase residue or can be the bond formed byhybridization between a probe and a template strand. Selectivelycleavable bonds are not limited to covalent bonds, and can benon-covalent bonds or associations, such as those based on hydrogenbonds, hydrophobic bonds, ionic bonds, pi-bond ring stackinginteractions, Van der Waals interactions, and the like.

“Moiety” is one of two or more parts into which something may bedivided, such as, for example, the various parts of a tether, a moleculeor a probe.

“Tether” or “tether member” refers to a polymer or molecular constructhaving a generally linear dimension and with an end moiety at each oftwo opposing ends. A tether is attached to a substrate with a linkage inat least one end moiety to form a substrate construct. The end moietiesof the tether may be connected to cleavable linkages to the substrate orcleavable intra-tether linkages that serve to constrain the tether in a“constrained configuration”. After the daughter strand is synthesized,each end moiety has an end linkage that couples directly or indirectlyto other tethers. The coupled tethers comprise the constrained Xpandomerthat further comprises the daughter strand. Tethers have a “constrainedconfiguration” and an “expanded configuration”. The constrainedconfiguration is found in substrate constructs and in the daughterstrand. The constrained configuration of the tether is the precursor tothe expanded configuration, as found in Xpandomer products. Thetransition from the constrained configuration to the expandedconfiguration results cleaving of selectively cleavable bonds that maybe within the primary backbone of the daughter strand or intra-tetherlinkages. A tether in a constrained configuration is also used where atether is added to form the daughter strand after assembly of the“primary backbone”. Tethers can optionally comprise one or morereporters or reporter constructs along its length that can encodesequence information of substrates. The tether provides a means toexpand the length of the Xpandomer and thereby lower the sequenceinformation linear density.

“Tether constructs” are tethers or tether precursors composed of one ormore tether segments or other architectural components for assemblingtethers such as reporter constructs, or reporter precursors, includingpolymers, graft copolymers, block copolymers, affinity ligands,oligomers, haptens, aptamers, dendrimers, linkage groups or affinitybinding group (e.g., biotin).

“Tether element” or “tether segment” is a polymer having a generallylinear dimension with two terminal ends, where the ends formend-linkages for concatenating the tether elements. Tether elements maybe segments of tether constructs. Such polymers can include, but are notlimited to: polyethylene glycols, polyglycols, polypyridines,polyisocyanides, polyisocyanates, poly(triarylmethyl)methacrylates,polyaldehydes, polypyrrolinones, polyureas, polyglycol phosphodiesters,polyacrylates, polymethacrylates, polyacrylamides, polyvinyl esters,polystyrenes, polyamides, polyurethanes, polycarbonates, polybutyrates,polybutadienes, polybutyrolactones, polypyrrolidinones,polyvinylphosphonates, polyacetamides, polysaccharides,polyhyaluranates, polyamides, polyimides, polyesters, polyethylenes,polypropylenes, polystyrenes, polycarbonates, polyterephthalates,polysilanes, polyurethanes, polyethers, polyamino acids, polyglycines,polyprolines, N-substituted polylysine, polypeptides, side-chainN-substituted peptides, poly-N-substituted glycine, peptoids, side-chaincarboxyl-substituted peptides, homopeptides, oligonucleotides,ribonucleic acid oligonucleotides, deoxynucleic acid oligonucleotides,oligonucleotides modified to prevent Watson-Crick base pairing,oligonucleotide analogs, polycytidylic acid, polyadenylic acid,polyuridylic acid, polythymidine, polyphosphate, polynucleotides,polyribonucleotides, polyethylene glycol-phosphodiesters, peptidepolynucleotide analogues, threosyl-polynucleotide analogues,glycol-polynucleotide analogues, morpholino-polynucleotide analogues,locked nucleotide oligomer analogues, polypeptide analogues, branchedpolymers, comb polymers, star polymers, dendritic polymers, random,gradient and block copolymers, anionic polymers, cationic polymers,polymers forming stem-loops, rigid segments and flexible segments.

“Peptide nucleic acid” or “PNA” is a nucleic acid analog havingnucleobase residues suitable for hybridization to a nucleic acid, butwith a backbone that comprises amino acids or derivatives or analogsthereof.

“Phosphono-peptide nucleic acid” or “pPNA” is a peptide nucleic acid inwhich the backbone comprises amino acid analogs, such asN-(2-hydroxyethyl)phosphonoglycine or N-(2-aminoethyl)phosphonoglycine,and the linkages between nucleobase units are through phosphonoester orphosphonoamide bonds.

“Serine nucleic acid” or “SerNA” is a peptide nucleic acid in which thebackbone comprises serine residues. Such residues can be linked throughamide or ester linkages.

“Hydroxyproline nucleic acid” or “HypNA” is a peptide nucleic acid inwhich the backbone comprises 4-hydroxyproline residues. Such residuescan be linked through amide or ester linkages.

“Reporter element” is a signaling element, molecular complex, compound,molecule or atom that is also comprised of an associated “reporterdetection characteristic”. Other reporter elements include, but are notlimited to, FRET resonant donor or acceptor, dye, quantum dot, bead,dendrimer, upconverting fluorophore, magnet particle, electron scatterer(e.g., boron), mass, gold bead, magnetic resonance, ionizable group,polar group, hydrophobic group. Still others are fluorescent labels,such as but not limited to, ethidium bromide, SYBR Green, Texas Red,acridine orange, pyrene, 4-nitro-1,8-naphthalimide, TOTO-1, YOYO-1,cyanine 3 (Cy3), cyanine 5 (Cy5), phycoerythrin, phycocyanin,allophycocyanin, FITC, rhodamine, 5(6)-carboxyfluorescein, fluorescentproteins, DOXYL (N-oxyl-4,4-dimethyloxazolidine), PROXYL(N-oxyl-2,2,5,5-tetramethylpyrrolidine), TEMPO(N-oxyl-2,2,6,6-tetramethylpiperidine), dinitrophenyl, acridines,coumarins, Cy3 and Cy5 (Biological Detection Systems, Inc.), erytrosine,coumaric acid, umbelliferone, texas red rhodaine, tetramethyl rhodamin,Rox, 7-nitrobenzo-1-oxa-1-diazole (NBD), oxazole, thiazole, pyrene,fluorescein or lanthamides; also radioisotopes (such as ³³P, ³H, ¹⁴C,³⁵S, ¹²⁵I, ³²P or ¹³¹I), ethidium, Europium, Ruthenium, and Samarium orother radioisotopes; or mass tags, such as, for example, pyrimidinesmodified at the C5 position or purines modified at the N7 position,wherein mass modifying groups can be, for examples, halogen, ether orpolyether, alkyl, ester or polyester, or of the general type XR, whereinX is a linking group and R is a mass-modifying group, chemiluminescentlabels, spin labels, enzymes (such as peroxidases, alkalinephosphatases, beta-galactosidases, and oxidases), antibody fragments,and affinity ligands (such as an oligomer, hapten, and aptamer).Association of the reporter element with the tether can be covalent ornon-covalent, and direct or indirect. Representative covalentassociations include linker and zero-linker bonds. Included are bonds tothe tether backbone or to a tether-bonded element such as a dendrimer orsidechain. Representative non-covalent bonds include hydrogen bonds,hydrophobic bonds, ionic bonds, pi-bond ring stacking, Van der Waalsinteractions, and the like. Ligands, for example, are associated byspecific affinity binding with binding sites on the reporter element.Direct association can take place at the time of tether synthesis, aftertether synthesis, and before or after Xpandomer synthesis.

A “reporter” is composed of one or more reporter elements. Reportersinclude what are known as “tags” and “labels.” The probe or nucleobaseresidue of the Xpandomer can be considered a reporter. Reporters serveto parse the genetic information of the target nucleic acid.

“Reporter construct” comprises one or more reporters that can produce adetectable signal(s), wherein the detectable signal(s) generally containsequence information. This signal information is termed the “reportercode” and is subsequently decoded into genetic sequence data. A reporterconstruct may also comprise tether segments or other architecturalcomponents including polymers, graft copolymers, block copolymers,affinity ligands, oligomers, haptens, aptamers, dendrimers, linkagegroups or affinity binding group (e.g., biotin).

“Reporter detection characteristic” referred to as the “signal”describes all possible measurable or detectable elements, properties orcharacteristics used to communicate the genetic sequence information ofa reporter directly or indirectly to a measurement device. Theseinclude, but are not limited to, fluorescence, multi-wavelengthfluorescence, emission spectrum fluorescence quenching, FRET, emission,absorbance, reflectance, dye emission, quantum dot emission, bead image,molecular complex image, magnetic susceptibility, electron scattering,ion mass, magnetic resonance, molecular complex dimension, molecularcomplex impedance, molecular charge, induced dipole, impedance,molecular mass, quantum state, charge capacity, magnetic spin state,inducible polarity, nuclear decay, resonance, or complementarity.

“Reporter Code” is the genetic information from a measured signal of areporter construct. The reporter code is decoded to providesequence-specific genetic information data.

“Xprobe” is an expandable oligomeric substrate construct. Each Xprobehas a probe member and a tether member. The tether member generallyhaving one or more reporter constructs. Xprobes with 5′-monophosphatemodifications are compatible with enzymatic ligation-based methods forXpandomer synthesis. Xprobes with 5′ and 3′ linker modifications arecompatible with chemical ligation-based methods for Xpandomer synthesis.

“Xmer” is an expandable oligomeric substrate construct. Each Xmer has anoligomeric substrate member and a tether member, the tether membergenerally having one or more reporter constructs. Xmers are5′-triphosphates compatible with polymerase-based methods forsynthesizing Xpandomers.

“RT-NTP” is an expandable, 5′ triphosphate-modified nucleotide substrateconstruct (“monomeric substrate”) compatible with template dependantenzymatic polymerization. An RT-NTP has a modified deoxyribonucleotidetriphosphate (“DNTP”), ribonucleotide triphosphate (“RNTP”), or afunctionally equivalent analog substrate, collectively referred to asthe nucleotide triphosphate substrate (“NTPS”). An RT-NTP has twodistinct functional components; namely, a nucleobase 5′-triphosphate anda tether or tether precursor. After formation of the daughter strand thetether is attached between each nucleotide at positions that allow forcontrolled RT expansion. In one class of RT-NTP (e.g., Class IX), thetether is attached after RT-NTP polymerization. In some cases, theRT-NTP has a reversible end terminator and a tether that selectivelycrosslinks directly to adjacent tethers. Each tether can be uniquelyencoded with reporters that specifically identify the nucleotide towhich it is tethered.

“XNTP” is an expandable, 5′ triphosphate modified nucleotide substratecompatible with template dependent enzymatic polymerization. An XNTP hastwo distinct functional components; namely, a nucleobase 5′-triphosphateand a tether or tether precursor that is attached within each nucleotideat positions that allow for controlled RT expansion by intra-nucleotidecleavage.

“Processive” refers to a process of coupling of substrates which isgenerally continuous and proceeds with directionality. While not boundby theory, both ligases and polymerases, for example, exhibit processivebehavior if substrates are added to a nascent daughter strandincrementally without interruption. The steps of hybridization andligation, or hybridization and polymerization, are not seen asindependent steps if the net effect is processive growth of the nascentdaughter strand. Some but not all primer-dependent processes areprocessive.

“Promiscuous” refers to a process of coupling of substrates thatproceeds from multiple points on a template at once, and is not primerdependent, and indicates that chain extension occurs in parallel(simultaneously) from more than one point of origin.

“Single-base extension” refers to a cyclical stepwise process in whichmonomeric substrates are added one by one. Generally the couplingreaction is restrained from proceeding beyond single substrate extensionin any one step by use of reversible blocking groups.

“Single-probe extension” refers to a cyclical stepwise process in whicholigomeric substrates are added one by one. Generally the couplingreaction is restrained from proceeding beyond single substrate extensionin any one step by use of reversible blocking groups.

“Corresponds to” or “corresponding” is used here in reference to acontiguous single-stranded sequence of a probe, oligonucleotide,oligonucleotide analog, or daughter strand that is complementary to, andthus “corresponds to”, all or a portion of a target nucleic acidsequence. The complementary sequence of a probe can be said tocorrespond to its target. Unless otherwise stated, both thecomplementary sequence of the probe and the complementary sequence ofthe target are individually contiguous sequences.

“Nuclease-resistant” refers to is a bond that is resistant to a nucleaseenzyme under conditions where a DNA or RNA phosphodiester bond willgenerally be cleaved. Nuclease enzymes include, but are not limited to,DNase I, Exonuclease III, Mung Bean Nuclease, RNase I, and RNase H. Oneskilled in this field can readily evaluate the relative nucleaseresistance of a given bond.

“Ligase” is an enzyme generally for joining 3′-OH 5′-monophosphatenucleotides, oligomers, and their analogs. Ligases include, but are notlimited to, NAD⁺-dependent ligases including tRNA ligase, Taq DNAligase, Thermus filiformis DNA ligase, Escherichia coli DNA ligase, TthDNA ligase, Thermus scotoductus DNA ligase, thermostable ligase,Ampligase thermostable DNA ligase, VanC-type ligase, 9° N DNA Ligase,Tsp DNA ligase, and novel ligases discovered by bioprospecting. Ligasesalso include, but are not limited to, ATP-dependent ligases including T4RNA ligase, T4 DNA ligase, T7 DNA ligase, Pfu DNA ligase, DNA ligase 1,DNA ligase III, DNA ligase IV, and novel ligases discovered bybioprospecting. These ligases include wild-type, mutant isoforms, andgenetically engineered variants.

“Polymerase” is an enzyme generally for joining 3′-OH 5′-triphosphatenucleotides, oligomers, and their analogs. Polymerases include, but arenot limited to, DNA-dependent DNA polymerases, DNA-dependent RNApolymerases, RNA-dependent DNA polymerases, RNA-dependent RNApolymerases, T7 DNA polymerase, T3 DNA polymerase, T4 DNA polymerase, T7RNA polymerase, T3 RNA polymerase, SP6 RNA polymerase, DNA polymerase 1,Klenow fragment, Thermophilus aquaticus DNA polymerase, Tth DNApolymerase, VentR® DNA polymerase (New England Biolabs), Deep VentR® DNApolymerase (New England Biolabs), Bst DNA Polymerase Large Fragment,Stoeffel Fragment, 9° N DNA Polymerase, 9° N DNA polymerase, Pfu DNAPolymerase, Tfl DNA Polymerase, Tth DNA Polymerase, RepliPHI Phi29Polymerase, Tli DNA polymerase, eukaryotic DNA polymerase beta,telomerase, Therminator™ polymerase (New England Biolabs), KOD HiFi™ DNApolymerase (Novagen), KOD1 DNA polymerase, Q-beta replicase, terminaltransferase, AMV reverse transcriptase, M-MLV reverse transcriptase,Phi6 reverse transcriptase, HIV-1 reverse transcriptase, novelpolymerases discovered by bioprospecting, and polymerases cited in US2007/0048748, U.S. Pat. No. 6,329,178, U.S. Pat. No. 6,602,695, and U.S.Pat. No. 6,395,524 (incorporated by reference). These polymerasesinclude wild-type, mutant isoforms, and genetically engineered variants.

“Encode” or “parse” are verbs referring to transferring from one formatto another, and refers to transferring the genetic information of targettemplate base sequence into an arrangement of reporters.

“Extragenetic” refers to any structure in the daughter strand that isnot part of the primary backbone; for example, an extragenetic reporteris not the nucleobase itself that lies in the primary backbone.

“Hetero-copolymer” is a material formed by combining differing units(e.g., monomer subunit species) into chains of a “copolymer”.Hetero-copolymers are built from discrete “subunit” constructs. A“subunit” is a region of a polymer composed a well-defined motif, whereeach motif is a species and carries genetic information. The termhetero-copolymer is also used herein to describe a polymer in which allthe blocks are blocks constructed of repeating motifs, each motif havingspecies-specific elements. The daughter strand and the Xpandomer areboth hetero-copolymers whereby each subunit motif encodes 1 or morebases of the target template sequence and the entire target sequence isdefined further with the sequence of motifs.

“Solid support” is a solid material having a surface for attachment ofmolecules, compounds, cells, or other entities. The surface of a solidsupport can be flat or not flat. A solid support can be porous ornon-porous. A solid support can be a chip or array that comprises asurface, and that may comprise glass, silicon, nylon, polymers,plastics, ceramics, or metals. A solid support can also be a membrane,such as a nylon, nitrocellulose, or polymeric membrane, or a plate ordish and can be comprised of glass, ceramics, metals, or plastics, suchas, for example, polystyrene, polypropylene, polycarbonate, orpolyallomer. A solid support can also be a bead, resin or particle ofany shape. Such particles or beads can be comprised of any suitablematerial, such as glass or ceramics, and/or one or more polymers, suchas, for example, nylon, polytetrafluoroethylene, TEFLON™, polystyrene,polyacrylamide, sepaharose, agarose, cellulose, cellulose derivatives,or dextran, and/or can comprise metals, particularly paramagneticmetals, such as iron.

“Reversibly blocking” or “terminator” refers to a chemical group thatwhen bound to a second chemical group on a moiety prevents the secondchemical group from entering into particular chemical reactions. A widerange of protecting groups are known in synthetic organic and bioorganicchemistry that are suitable for particular chemical groups and arecompatible with particular chemical processes, meaning that they willprotect particular groups during those processes and may be subsequentlyremoved or modified (see, e.g., Metzker et al. Nucleic Acids Res.,22(20): 4259,1994).

“Linker” is a molecule or moiety that joins two molecules or moieties,and provides spacing between the two molecules or moieties such thatthey are able to function in their intended manner. For example, alinker can comprise a diamine hydrocarbon chain that is covalently boundthrough a reactive group on one end to an oligonucleotide analogmolecule and through a reactive group on another end to a solid support,such as, for example, a bead surface. Coupling of linkers to nucleotidesand substrate constructs of interest can be accomplished through the useof coupling reagents that are known in the art (see, e.g., Efimov etal., Nucleic Acids Res. 27: 4416-4426, 1999). Methods of derivatizingand coupling organic molecules are well known in the arts of organic andbioorganic chemistry. A linker may also be cleavable or reversible.

General Overview

In general terms, methods and corresponding devices and products aredescribed for replicating single-molecule target nucleic acids. Suchmethods utilize “Xpandomers” which permit sequencing of the targetnucleic acid with increased throughput and accuracy. An Xpandomerencodes (parses) the nucleotide sequence data of the target nucleic acidin a linearly expanded format, thereby improving spatial resolution,optionally with amplification of signal strength. These processes arereferred to herein as “Sequencing by Expansion” or “SBX”.

As shown in FIG. 1A, native duplex nucleic acids have an extremelycompact linear data density; about a 3.4 Å center-to-center separationbetween sequential stacked bases (2) of each strand of the double helix(1), and are therefore tremendously difficult to directly image orsequence with any accuracy and speed. When the double-stranded form isdenatured to form single stranded polynucleotides (3,4), the resultingbase-to-base separation distances are similar, but the problem becomescompounded by domains of secondary structure.

As shown in FIG. 1B, Xpandomer (5), here illustrated as a concatenationof short oligomers (6,7) held together by extragenetic tethers T (8,9),is a synthetic replacement or “surrogate” for the nucleic acid target tobe sequenced. Bases complementary to the template are incorporated intothe Xpandomer, but the regularly spaced tethers serve to increase thedistance between the short oligomers (here each shown with fournucleobases depicted by circles). The Xpandomer is made by a process inwhich a synthetic duplex intermediate is first formed by replicating atemplate strand. The daughter strand is unique in that it has both alinear backbone formed by the oligomers and a constrained Xpandomerbackbone comprised of folded tethers. The tethers are then opened up or“expanded” to transform the product into a chain of elongated tethers.Figuratively, the daughter strand can be viewed as having twosuperimposed backbones: one linear (primary backbone) and the other with“accordion” folds (constrained Xpandomer). Selective cleavage of bondsin the daughter strand allows the accordion folds to expand to producean Xpandomer product. This process will be explained in more detailbelow, but it should be noted that the choice of four nucleobases peroligomer and particulars of the tether as shown in FIG. 1B is forpurpose of illustration only, and in no way should be construed to limitthe invention.

The separation distance “D” between neighboring oligomers in theXpandomer is now a process-dependent variable and is determined by thelength of the tether T. As will be shown, the length of the tether T isdesigned into the substrate constructs, the building blocks from whichthe Xpandomer is made. The separation distance D can be selected to begreater than 0.5 nm, or greater than 2 nm, or greater than 5 nm, orgreater than 10 nm, or greater than 50 nm, for example. As theseparation distance increases, the process of discriminating or“resolving” the individual oligomers becomes progressively easier. Thiswould also be true if, instead of oligomers, individual nucleobases ofanother Xpandomer species were strung together on a chain of tethers.

Referring again to FIG. 1A, native DNA replicates by a process ofsemi-conservative replication; each new DNA molecule is a “duplex” of atemplate strand (3) and a native daughter strand (4). The sequenceinformation is passed from the template to the native daughter strand bya process of “template-directed synthesis” that preserves the geneticinformation inherent in the sequence of the base pairs. The nativedaughter strand in turn becomes a template for a next generation nativedaughter strand, and so forth. Xpandomers are formed by a similarprocess of template-directed synthesis, which can be an enzymatic or achemical coupling process. However, unlike native DNA, once formed,Xpandomers cannot be replicated by a biological process ofsemi-conservative replication and are not suitable for amplification byprocesses such as PCR. The Xpandomer product is designed to limitunwanted secondary structure.

FIGS. 2A through 2D show representative Class I Xpandomer substrates(20,21,22,23). These are the building blocks from which Xpandomers aresynthesized. Other Xpandomer substrates (ten classes are disclosedherein) are addressed in the subsequent sections. The Xpandomersubstrate constructs shown here have two functional components; namely,a probe member (10) and a “tether” member (11) in a loop configuration.The loop forms the elongated tether “T” of the final product. Solely forconvenience in explanation, the probe member is again depicted with fournucleobase residues (14,15,16,17) as shown in FIG. 2B.

These substrate constructs can be end modified with R— groups, forexample as a 5′-monophosphate, 3′-OH suitable for use with a ligase(herein termed an “Xprobe”) or as a 5′-triphosphate, 3′-OH suitable foruse with a polymerase (herein termed an “Xmer”). Other R groups may beof use in various protocols. In the first example shown in FIG. 3B, wepresent the use of Xprobes in synthesis of an Xpandomer from a templatestrand of a target nucleic acid by a ligase-dependent process.

The four nucleobase residues (14,15,16,17) of the probe member (10) areselected to be complementary to a contiguous sequence of fournucleotides of the template. Each “probe” is thus designed to hybridizewith the template at a complementary sequence of four nucleotides. Bysupplying a library of many such probe sequences, a contiguouscomplementary replica of the template can be formed. This daughterstrand is termed an “Xpandomer intermediate”. Xpandomer intermediateshave duplex or single-stranded forms.

The tether loop is joined to the probe member (10) at the second andthird nucleobase residues (15,16). The second and third nucleobaseresidues (15,16) are also joined to each other by a “selectivelycleavable bond” (25) depicted by a “V”. Cleavage of this cleavable bondenables the tether loop to expand. The linearized tether can be said to“bridge” the selectively cleavable bond site of the primarypolynucleotide backbone of a daughter strand. Cleaving these bondsbreaks up the primary backbone and forms the longer Xpandomer.

Selective cleavage of the selectively cleavable bonds (25) can be donein a variety of ways including, but not limited to, chemical cleavage ofphosphorothiolate bonds, ribonuclease digestion of ribosyl 5′-3′phosphodiester linkages, cleavage of photocleavable bonds, and the like,as discussed is greater detail below.

The substrate construct (20) shown in FIG. 2A has a single tethersegment, represented here by an ellipse (26), for attachment of reporterelements. This segment is flanked with spacer tether segments (12,13),all of which collectively form the tether construct. One to manydendrimer(s), polymer(s), branched polymer(s) or combinations thereincan be used, for example, to construct the tether segment. For thesubstrate construct (21) of FIG. 2B, the tether construct is composed ofthree tether segments for attachment of reporter elements (27,28,29),each of which is flanked with a spacer tether segment. The combinationof reporter elements collectively form a “reporter construct” to producea unique digital reporter code (for probe sequence identification).These reporter elements include, but are not limited to, fluorophores,FRET tags, beads, ligands, aptamers, peptides, haptens, oligomers,polynucleotides, dendrimers, stem-loop structures, affinity labels, masstags, and the like. The tether loop (11) of the substrate construct (22)in FIG. 2C is “naked”. The genetic information encoded in this constructis not encoded on the tether, but is associated with the probe (10), forexample, in the form of tagged nucleotides. The substrate construct (23)of FIG. 2D illustrates the general principal: as indicated by theasterisk (*), the sequence information of the probe is encoded or“parsed” in the substrate construct in a modified form more readilydetected in a sequencing protocol. Because the sequence data isphysically better resolved after cleavage of the selectively cleavablebond (25) to form the linearly elongated Xpandomer polymer, the asterisk(*) represents any form of encoded genetic information for which this isa benefit. The bioinformatic element or elements (*) of the substrateconstruct, whatever their form, can be detectable directly or can beprecursors to which detectable elements are added in a post-assemblylabeling step. In some instances, the genetic information is encoded ina molecular property of the substrate construct itself, for example amulti-state mass tag. In other instances, the genetic information isencoded by one or more fluorophores of FRET donor:acceptor pairs, or ananomolecular barcode, or a ligand or combination of ligands, or in theform of some other labeling technique drawn from the art. Variousembodiments will be discussed in more detail below.

The tether generally serves a number of functions: (1) to sequentiallylink, directly or indirectly, to adjacent tethers forming the Xpandomerintermediate; (2) to stretch out and expand to form an elongated chainof tethers upon cleavage of selected bonds in the primary backbone orwithin the tether (see FIG. 1B); and/or (3) to provide a molecularconstruct for incorporating reporter elements, also termed “tags” or“labels”, that encode the nucleobase residue sequence information of itsassociated substrate. The tether can be designed to optimize theencoding function by adjusting spatial separations, abundance,informational density, and signal strength of its constituent reporterelements. A broad range of reporter properties are useful for amplifyingthe signal strength of the genetic information encoded within thesubstrate construct. The literature directed to reporters, molecular barcodes, affinity binding, molecular tagging and other reporter elementsis well known to one skilled in this field.

It can be seen that if each substrate of a substrate construct containsx nucleobases, then a library representing all possible sequentialcombinations of x nucleobases would contain 4^(x) probes (when selectingthe nucleobases from A, T, C or G). Fewer or more combinations can beneeded if other bases are used. These substrate libraries are designedso that each substrate construct contains (1) a probe (or at least onenucleobase residue) complementary to any one of the possible targetsequences of the nucleic acid to be sequenced and (2) a unique reporterconstruct that encodes the identity of the target sequence which thatparticular probe (or nucleobase) is complementary to. A library ofprobes containing two nucleobases would have 16 unique members; alibrary of probes containing three nucleobases would have 64 uniquemembers, and so forth. A representative library would have the fourindividual nucleobases themselves, but configured to accommodate atethering means.

Synthesis of an Xpandomer is illustrated in FIGS. 3A through 3C. Thesubstrate depicted here is an Xprobe and the method can be described ashybridization with primer-dependent processive ligation in freesolution.

Many well known molecular biological protocols, such as protocols forfragmenting the target DNA and ligating end adaptors, can be adapted foruse in sequencing methods and are used here to prepare the target DNA(30) for sequencing. Here we illustrate, in broad terms that would befamiliar to those skilled in the art, processes for polishing the endsof the fragments and blunt-ended ligation of adaptors (31,32) designedfor use with sequencing primers. These actions are shown in Step I ofFIG. 3A. In Steps II and III, the target nucleic acid is denatured andannealed with suitable primers (33) complementary to the adaptors.

In FIG. 3B, the primed template strand of Step III is contacted with alibrary of substrate constructs (36) and ligase (L), and in Step IVconditions are adjusted to favor hybridization followed by ligation at afree 3′-OH of a primer-template duplex. Optionally in Step V the ligasedissociates, and in Steps VI and VII, the process of hybridization andligation can be recognized to result in extension by cumulative additionof substrates (37,38) to the primer end. Although priming can occur fromadaptors at both ends of a single stranded template, the growth of anascent Xpandomer daughter strand is shown here to proceed from a singleprimer, solely for simplicity. Extension of the daughter strand isrepresented in Steps VI and VII, which are continuously repeated(incrementally, without interruption). These reactions occur in freesolution and proceed until a sufficient amount of product has beensynthesized. In Step VIII, formation of a completed Xpandomerintermediate (39) is shown.

Relatively long lengths of contiguous nucleotide sequence can beefficiently replicated in this manner to form Xpandomer intermediates.It can be seen that continuous read lengths (“contigs”) corresponding tolong template strand fragments can be achieved with this technology. Itwill be apparent to one skilled in the art that billions of these singlemolecule SBX reactions can be done simultaneously in an efficient batchprocess in a single tube. Subsequently, the shotgun products of thesesyntheses can be sequenced.

In FIG. 3C, the next steps of the SBX process are depicted. Step IXshows denaturation of the duplex Xpandomer intermediate followed bycleavage of selectively cleavable bonds in the backbone, with theselectively cleavable bonds designed so that the tether loops “open up”,forming the linearly elongated Xpandomer product (34). Such selectivecleavage may be achieved by any number of techniques known to oneskilled in the art, including, but not limited to, phosphorothiolatecleavage with metal cations as disclosed by Mag et al. (“Synthesis andselective cleavage of an oligodeoxynucleotide containing a bridgedinternucleotide 5′-phosphorothioate linkage”, Nucleic Acids Research19(7):1437-1441, 1991), acid catalyzed cleavage of phosphoramidate asdisclosed by Mag et al. (“Synthesis and selective cleavage ofoligodeoxyribonucleotides containing non-chiral internucleotidephosphoramidate linkages”, Nucleic Acids Research 17(15): 5973-5988,1989), selective nuclease cleavage of phosphodiester linkages asdisclosed by Gut et al. (“A novel procedure for efficient genotyping ofsingle nucleotide polymorphisms”, Nucleic Acids Research 28(5): E13,2000) and separately by Eckstein et al. (“Inhibition of restrictionendonuclease hydrolysis by phosphorothioate-containing DNA”, NucleicAcids Research, 25; 17(22): 9495,1989), and selective cleavage ofphotocleavable linker modified phosphodiester backbone as disclosed bySauer et al. (“MALDI mass spectrometry analysis of single nucleotidepolymorphisms by photocleavage and charge-tagging”, Nucleic AcidsResearch 31, 11 e63, 2003), Vallone et al. (“Genotyping SNPs using aUV-photocleavable oligonucleotide in MALDI-TOF MS”, Methods Mol. Bio.297:169-78, 2005), and Ordoukhanian et al. (“Design and synthesis of aversatile photocleavable DNA building block, application tophototriggered hybridization”, J. Am. Chem. Soc. 117, 9570-9571,1995).

Refinements of the basic process, such as wash steps and adjustment ofconditions of stringency are well within the skill of an experiencedmolecular biologist. Variants on this process, include for exampleimmobilization and parsing of the target strands, stretching and othertechniques to reduce secondary structure during synthesis of theXpandomer, post-expansion labeling, end-functionalization, andalternatives to ligase for linking the substrates will be discussed inthe materials that follow.

Synthesis of Xpandomers is done to facilitate the detection andsequencing of nucleic acids, and is applicable to nucleic acids of allkinds. The process is a method for “expanding” or “elongating” thelength of backbone elements (or subunits) encoding the sequenceinformation (expanded relative to the small nucleotide-to-nucleotidedistances of native nucleic acids) and optionally also serves toincrease signal intensity (relative to the nearly indistinguishable,low-intensity signals observed for native nucleotides). As such, thereporter elements incorporated in the expanded synthetic backbone of anXpandomer can be detected and processed using a variety of detectionmethods, including detection methods well known in the art (for example,a CCD camera, an atomic force microscope, or a gated mass spectrometer),as well as by methods such as a massively parallel nanopore sensorarray, or a combination of methods. Detection techniques are selected onthe basis of optimal signal to noise, throughput, cost, and likefactors.

Turning to FIG. 4, a simple model of a detection technology is shown;namely, a nanopore (40) with FRET donor (42) in a membrane (44), whichis excited by light of λ₁ wavelength. As the Xpandomer product (41)elongates and is conveyed through the nanopore (40) in the direction ofarrow (45), serial bursts of emission of wavelength λ₂ from excitedfluorophores, in the proximity of the pore, are detected. The emissionwavelengths (λ₂) are temporally spaced as a function of the length ofthe tether and the speed of the Xpandomer passing through the nanopore.By capturing these analog signals and digitally processing them, thesequence information can be read directly from the Xpandomer. It shouldbe noted that in this detection method, the nanopore and membrane canhave many paths through which the Xpandomer may translocate. The FRETdetection requires there be at least one excited FRET donor along eachpath. In contrast, a Coulter counter based nanopore can only have anadditional translocation hole at the cost of signal-to-noise.

In the nanopore-based detection technique of FIG. 4, which depictschains of Xprobes of the structure shown in FIG. 2B, the tetherconstructs contain multi-element reporter constructs, as indicated bythe box-like reporter members (27,28,29) disposed along the tether.Relevant nanopore sequencing technology is disclosed, for example, byBranton et al. in U.S. Pat. No. 6,627,067 and by Lee et al. (Lee, J Wand A Meller. 2007. Rapid sequencing by direct nanoscale reading ofnucleotide bases in individual DNA chains. In “New High ThroughputTechnologies for DNA Sequencing and Genomics, 2”, Elsevier).

FIG. 5 demonstrates how multi-element reporter constructs, herecomprised of FRET acceptor fluorophores, appear to a detector positionedat the FRET gate. It can be seen in this multi-channel plot ofemissions, that analog signals are generated at generally regular timeintervals and can be parsed as a type of digital code (here termed areporter code, and, for this example, an Xprobe ID) revealing theidentity and order of the Xprobe subunits and thus the genetic sequenceof the illustrated Xpandomer. Various combinations of reporters can beused to create a library of reporter codes that sequentially encode any4-base combination of A, T, G or C of the described Xprobe. In thisexample, combinations of three fluorophores are used to producetwenty-two reporter codes. In this way, the sequence ACTG is seen to befollowed by GCCG; followed by AAAT. Vertically placed dotted linesseparate the fluorimetry data and the corresponding subunits of theXpandomer (shown schematically). An interpretive algorithm immediatelybelow the plot shows how the regularly spaced analog signals aretransformed into a readable genetic sequence.

The Xprobe substrate construct illustrated in FIG. 5, which uses amulti-element tether construct composed of three reporter labeledsegments, each of which is flanked with spacer tether segments, toencode the sequence identity of the substrate, is further elaborated.The first tether segment is reporter code #1 (reading from left toright), and is read as a high signal in the red channel. The secondtether segment is reporter code #9, and is read as a high green signaland a low red signal. The third tether segment is reporter code #8, andis read as a low blue signal and a low red signal. Reporter code #1 isused as a clock or synchronization signal; reporter code #9 encodes thefirst probe moiety “AC”; reporter code number #8 encodes the secondmoiety “TG” of the probe. Taken together, the sequential reporter codeof “1-9-8” corresponds to a particular species of Xprobe (Xprobe ID117), which in turn corresponds by design to the sequence fragment“ACTG”. Three Xprobe IDs encode the entire contiguous sequence shown inthe plot, “ACTGGCCGAAAT”. The fluorophore emissions, the table fordecoding reporter codes and sequence fragments, and the correspondingphysical representations of the reporter constructs are separated by thedotted lines of the figures according to structural subunits of theXpandomer so that it can readily be seen how the sequence information isdecoded and digitized.

FIG. 6 is a table of fluorophore labels from which the example of FIG. 5was prepared. This illustrates more generally the use of combinations ofmulti-state reporter codes to parse information in the form ofdetectable signals. Fluorophores having twenty-two possible emissionstates are used to form the reporter constructs of this example. Threefluorophore labels per oligomer are more than adequate to code allpossible 4mer combinations of A, T, C and G. By increasing the length ofthe tether, the resolution between the fluorophore label emissions isimproved, benefiting the accuracy of the detection step, a principlethat is generally applicable.

Reporters useable with tether constructs of this kind are of many types,not merely fluorophores, and can be measured using a corresponding broadrange of high throughput and accurate detection technologies,technologies that might not otherwise be useful to sequence nativenucleic acids because of limited resolution. Massively parallel, stateof the art detection methods, such as nanopore sensor arrays, arefacilitated by the more measurable characteristics of Xpandomers.Inefficiencies in sequencing detection processes can be reduced bypre-purifying batches of Xpandomers to eliminate incomplete or shortreaction products. Methods for end-modifying synthesized Xpandomers areprovided that can be utilized for both purification and as a means offacilitating Xpandomer presentation to the detector. Furthermore, thereading process is not constrained by limitation to capping, uncapping,nucleotide extension, labeling, or other concurrent processing methods.

FIG. 7A describes a partial duplex template designed with a twenty base5′ overhang to demonstrate processive ligation of substrates andprimer-initiated template-directed ligation in free solution. FIG. 7B isa photograph of a gel demonstrating ligation of the substrates using theprimer-template format described in FIG. 7A. For this example,dinucleotide oligomeric substrates of the sequence 5′ phosphate CA 3′are hybridized to the template in the presence of a primer and T4 DNAligase. The unduplexed end-overhang (if any) is then nuclease digestedand the ligation products are separated on a 20% acrylamide gel. Theligation results in product polymers containing demonstrably ligatedsubunits. As indicated by the banding pattern, the ligase positivereactions run out in lanes 1, 3, 5, 7 and 9, which contain progressivelylonger templates (4, 8, 12, 16, and 20 bases, respectively), clearlydemonstrate sequential ligation of 2mer substrates (increased lengths ofexonuclease protected duplexes). Lanes 2, 4, 6, 8 and 10 are negativecontrols containing no ligase and show complete exonuclease digestion ofunligated products.

FIG. 7C is a second gel showing template-directed ligation ofsubstrates. Four progressively longer positive control templates, againduplexed with an extension primer, were assayed (4, 8, 12, and 16template bases, respectively). Again, dinucleotide oligomeric substratesof the sequence 5′ phosphate CA 3′ are hybridized to the template in thepresence of a primer and T4 DNA ligase. The unduplexed end-overhang (ifany) is then nuclease digested and the ligation products are separatedon a 20% acrylamide gel. Oligomeric substrates (again 2mers) are seen toligate to the template in lanes 1, 2, 3 and 4, but not in lanes 5 and 6,where the template strands contain a mismatch with the 5′ (phosphate) CA3′ dinucleotide (Lane 5 template—5′ CGCG 3′; Lane 6 template—5′ GGGG3′).

The gel results shown in FIG. 7D demonstrate multiple, template-directedligations of a Bis(aminomodified) tetranucleotide probe. The aliphaticamino modifiers were of the linkage and composition described in FIG.26. For this example, a tetranucleotide oligomeric substrate of thesequence 5′ (phosphate) C (amino) A (amino) C A 3′ was hybridized to arange of progressively longer complimentary templates (duplexed with anextension primer) in the presence of a primer and T4 DNA ligase. Theunduplexed end-overhang (if any) was then nuclease digested and theligation products are separated on a 20% acrylamide gel. The ligationresults in product polymers containing demonstrably ligated subunits.Lanes 1 and 2 represent 16mer and 20mer size controls. Lanes 3, 4, 5, 6,7, 8, and 9 show ligation products for progressively longercomplementary templates (4, 6, 8, 12, 16, 18, and 20 template bases,respectively). Multiple tetramer ligations are observed for longertemplates reactions (Lanes 6-9). Lane 10 shows essentially completeligase inhibition due to template-probe mismatch (template—5′ CGCG 3′).

The gel results shown in FIG. 7E demonstrate multiple, template-directedligations of a Bis(aminomodified) hexanucleotide probe. The aliphaticamino modifiers were of the linkage and composition described in FIG.26. For this example, a hexanucleotide oligomeric substrate of thesequence 5′ (phosphate) C A (amino) C (amino) A C A 3′ was hybridized toa range of progressively longer complimentary templates (duplexed withan extension primer) in the presence of a primer and T4 DNA ligase. Theunduplexed end-overhang (if any) was then nuclease digested and theligation products are separated on a 20% acrylamide gel. The ligationresults in product polymers containing demonstrably ligated subunits.Lanes 1 and 2 represent 16mer and 20mer size controls. Lanes 3, 4, 5, 6,7, 8 and 9 show ligation products for progressively longer complementarytemplates (4, 6, 8, 12, 16, 18, and 20 template bases, respectively).Multiple tetramer ligations are observed for longer templates reactions(Lanes 5-9). Lane 10 shows nearly complete ligase inhibition due totemplate-probe mismatch (template—5′ CGCGCG 3′).

Substrates include both probe members (i.e., oligomers astemplate-specific binding members for assembling the Xpandomerintermediate), and monomers (i.e., individual nucleobase members as thetemplate-specific binding elements). We term the first “probe-type”substrates and the second “monomer-type” substrates. As illustrated inFIG. 8, probe-type Xpandomers have five basic subgenera, while FIG. 9illustrates five basic subgenera of monomer-type Xpandomers. The tablesof FIGS. 8 and 9 include three columns: the first describing substrateconstructs, the second Xpandomer intermediates, and the third theXpandomer products characteristic of the subgenus (by row). The tablesare provided here as an overview, with methods making and using the samebeing disclosed in greater detail herein below. In FIGS. 8 and 9, “P”refers to a probe member, “T” to a tether member (or loop tether ortether arm precursor), “N” to a monomer (an individual nucleobase ornucleobase residue), and “R” to an end group.

More specifically, in the table of FIG. 8 the following nomenclature isused:

-   -   P is a probe substrate member and is composed of P¹—P², where P¹        is a first probe moiety and P² is a second probe moiety;    -   T is a tether;    -   Brackets indicate a subunit of the daughter strand, wherein each        subunit is a subunit motif having a species-specific probe        member, further wherein said probe members of said subunit        motifs are serially complementary to the corresponding        contiguous nucleotide sequence of the template strand, denoted        here P^(1′)—P^(2′), and form a primary backbone of the Xpandomer        intermediate, and wherein the tether members, optionally in        combination with the probe moieties, form a constrained        Xpandomer backbone. Cleavage of one or more selectively        cleavable bonds within the Xpandomer intermediate enables        expansion of the subunits to produce an Xpandomer product, the        subunits of which are also indicated with brackets;    -   α denotes a species of subunit motif selected from a library of        subunit motifs;    -   ε is a first linker group attached to a first terminus or moiety        of a probe member or tether; under controlled conditions, is        capable of selectively reacting with, directly or via        crosslinkers, linker group 6 of an abutting terminus of an        adjacent subunit to form covalent or equivalently durable        linkages;    -   δ is a second linker group attached to a first terminus or        moiety of a probe member or tether; under controlled conditions,        6 is capable of selectively reacting with, directly or via        crosslinkers, linker group of an abutting terminus of an        adjacent subunit to form covalent or equivalently durable        linkages;    -   χ represents a bond with an adjacent subunit and is the product        linkage of the reaction of linker groups δ and ε;    -   ˜ denotes a selectively cleavable bond, which may be the same or        different when multiple selectively cleavable bonds are present;    -   R¹ includes, but is not limited to, hydroxyl, hydrogen,        triphosphate, monophosphate, ester, ether, glycol, amine, amide,        and thioester;    -   R² includes, but is not limited to, hydroxyl, hydrogen,        triphosphate, monophosphate, ester, ether, glycol, amine, amide,        and thioester; and    -   κ denotes the κ^(th) subunit in a chain of m subunits, where        κ=1, 2, . . . to m, where m>3, and generally m>20, and        preferably m>50, and more preferentially m>1000.

More specifically, and in context of the table FIG. 9, the followingnomenclature is used:

-   -   N is a nucleobase residue;    -   T is a tether;    -   Brackets indicate a subunit of the daughter strand, wherein each        subunit is a subunit motif having a species-specific nucleobase        residue, further wherein said nucleobase residues of said        subunit motifs are serially complementary to the corresponding        contiguous nucleotide sequence of the template strand, denoted        here N′, and form a primary backbone of the Xpandomer        intermediate, and wherein the tether members, optionally in        combination with the nucleobase residues, form a constrained        Xpandomer backbone. Cleavage of one or more selectively        cleavable bonds within the Xpandomer intermediate enables        expansion of the subunits to produce an Xpandomer product, the        subunits of which are also indicated with brackets;    -   n¹ is a first portion of a nucleobase residue;    -   n2 is a second portion of a nucleobase residue;    -   ε is a first linker group attached to a first terminus or moiety        of a probe member or tether; under controlled conditions, ε is        capable of selectively reacting with, directly or via        crosslinkers, linker group δ of an abutting terminus of an        adjacent subunit to form covalent or equivalently durable        linkages;    -   δ is a second linker group attached to a first terminus or        moiety of a probe member or tether; under controlled conditions,        δ is capable of selectively reacting with, directly or via        crosslinkers, linker group ε of an abutting terminus of an        adjacent subunit to form covalent or equivalently durable        linkages;    -   χ represents a bond with an adjacent subunit and is the product        linkage of the reaction of linkage groups δ and ε;    -   χ¹ is the product linkage of the reaction of linkage groups δ¹        and ε¹;    -   χ² is the product linkage of the reaction of linkage groups δ²        and ε²;    -   ˜ denotes a selectively cleavable bond, which may be the same or        different when multiple selectively cleavable bonds are present;    -   R¹ includes, but is not limited to, hydroxyl, hydrogen,        triphosphate, monophosphate, ester, ether, glycol, amine, amide,        and thioester;    -   R² includes, but is not limited to, hydroxyl, hydrogen,        triphosphate, monophosphate, ester, ether, glycol, amine, amide,        and thioester; and    -   κ denotes the κ^(th) subunit in a chain of m subunits, where        κ=1, 2, . . . to m, where m>10, and generally m>50, and        typically m>500 or >5,000.

Oligomeric Constructs

Xpandomer precursors and constructs can be divided into two categoriesbased upon the substrate (oligomeric or monomeric) used for templatedirected assembly. The Xpandomer structure, precursors and synthesismethods for those based upon the oligomer substrates are discussedbelow.

The substrate constructs are reagent precursors to the Xpandomer andgenerally have a tether member and a substrate. The substrate discussedhere is an oligomer substrate or probe, generally made up of a pluralityof nucleobase residues. By generating combinatorial-type libraries oftwo to twenty nucleobase residues per probe, generally 2 to 10 andtypically 2, 3, 4, 5 or 6 nucleobase residues per probe, probe librariesuseful as reagents in the synthesis of Xpandomers precursors (substrateconstructs) are generated.

The probe is generally described below as having two probe moieties, P¹and P². These probe moieties are generally depicted in the figures asdinucleotides, but in general P¹ and P² have each at least onenucleobase residue. In the example of a probe with two nucleobaseresidues, the probe moieties P¹ and P² would be single nucleobaseresidues. The number of nucleobase residues for each is chosen,appropriately, for the Xpandomer synthesis method and may not be equalin P¹ and P².

For the substrate constructs where ε and δ linker groups are used tocreate inter-subunit linkages, a broad range of suitable commerciallyavailable chemistries (Pierce, Thermo Fisher Scientific, USA) can beadapted for this purpose. Common linker chemistries include, forexample, NHS-esters with amines, maleimides with sulfhydryls,imidoesters with amines, EDC with carboxyls for reactions with amines,pyridyl disulfides with sulfhydryls, and the like. Other embodimentsinvolve the use of functional groups like hydrazide (HZ) and4-formylbenzoate (4FB) which can then be further reacted to formlinkages. More specifically, a wide range of crosslinkers (hetero- andhomo-bifunctional) are broadly available (Pierce) which include, but arenot limited to, Sulfo-SMCC (Sulfosuccinimidyl4-[N-maleimidomethyl]cyclohexane-1-carboxylate), SIA (N-Succinimidyliodoacetate), Sulfo-EMCS ([N-e-Maleimidocaproyloxy]sulfosuccinimideester), Sulfo-GMBS (N-[g-Maleimido butyryloxy]sulfosuccinimide ester),AMAS N-(a-Maleimidoacetoxy) succinimide ester), BMPS (N EMCA(N-e-Maleimidocaproic acid)-[β-Maleimidopropyloxy]succinimide ester),EDC (1-Ethyl-3-[3-dimethylaminopropyl]carbodiimide Hydrochloride),SANPAH (N-Succinimidyl-6-[4′-azido-2′-nitrophenylamino]hexanoate), SADP(N-Succinimidyl(4-azidophenyl)-1,3′-dithiopropionate), PMPI(N-[p-Maleimidophenyl]isocy, BMPH (N-[β-Maleimidopropionicacid]hydrazide, trifluoroacetic acid salt) anate), EMCH([N-e-Maleimidocaproic acid]hydrazide, trifluoroacetic acid salt), SANH(succinimidyl 4-hydrazinonicotinate acetone hydrazone), SHTH(succinimidyl 4-hydrazidoterephthalate hydrochloride), and C6-SFB(C6-succinimidyl 4-formylbenzoate). Also, the method disclosed byLetsinger et al. (“Phosphorothioate oligonucleotides having modifiedinternucleoside linkages”, U.S. Pat. No. 6,242,589) can be adapted toform phosphorothiolate linkages.

Further, well established protection/deprotection chemistries arebroadly available for common linker moieties (Benoiton, “Chemistry ofPeptide Synthesis”, CRC Press, 2005). Amino protection include, but arenot limited to, 9-Fluorenylmethyl carbamate (Fmoc-NRR′), t-Butylcarbamate (Boc-NRR′), Benzyl carbamate (Z-NRR′, Cbz-NRR′), AcetamideTrifluoroacetamide, Phthalimide, Benzylamine (Bn-NRR′),Triphenylmethylamine (Tr-NRR′), and Benzylideneaminep-Toluenesulfonamide (Ts-NRR′). Carboxyl protection include, but are notlimited to, Methyl ester, t-Butyl ester, Benzyl ester, S-t-Butyl ester,and 2-Alkyl-1,3-oxazoline. Carbonyl include, but are not limited to,Dimethyl acetal 1,3-Dioxane, and 1,3-Dithiane N,N-Dimethylhydrazone.Hydroxyl protection include, but are not limited to, Methoxymethyl ether(MOM-OR), Tetrahydropyranyl ether (THP-OR), t-Butyl ether, Allyl ether,Benzyl ether (Bn-OR), t-Butyldimethylsilyl ether (TBDMS-OR),t-Butyldiphenylsilyl ether (TBDPS-OR), Acetic acid ester, Pivalic acidester, and Benzoic acid ester.

While the tether is often depicted as a reporter construct with threereporter groups, various reporter configurations can be arrayed on thetether, and can comprise single reporters that identify probeconstituents, single reporters that identify probe species, molecularbarcodes that identify probe species, or the tether may be naked polymer(having no reporters). In the case of the naked polymer, the reportersmay be the probe itself, or may be on a second tether attached to theprobe. In some cases, one or more reporter precursors are arrayed on thetether, and reporters are affinity bound or covalently bound followingassembly of the Xpandomer product.

As discussed above, FIG. 8 provides an overview of oligomeric constructsof the invention, with five classes being distinguished: Classes I, II,III, IV, and V. These classes apply to both Xprobes and Xmers. Eachclass will be discussed individually below.

Class I Oligomeric Constructs

Turning to FIG. 10, Class I oligomeric constructs are described in moredetail. FIGS. 10A through 10C employ a notation adapted for showingthese molecules as substrates and as hetero-copolymer products of theSBX process. The figures are read from left to right, showing first theprobe substrate construct (oligomeric precursor of Xpandomer), then theintermediate duplex daughter strand in the center, and on the right theXpandomer product prepared for sequencing.

As shown in FIG. 10A, a Class I substrate construct has an oligomericprobe member (—P¹˜P²—) (100) and a tether member, T (99). The tether isattached by two end linkages (108,109) to probe moieties P¹ and P².These constraints prevent the tether from elongating or expanding andthus in a constrained configuration. Under template-directed assembly,substrates form a duplex with the target template such that thesubstrates are abutted.

R¹ and R² are end groups configured as appropriate for the synthesisprotocol in which the substrate construct is used. For example,R¹=5′-phosphate and R²=3′-OH, would find use in a ligation protocol, andR¹=5′-triphosphate and R²=3′-OH for a polymerase protocol. Optionally,R² can be configured with a reversible blocking group for cyclicalsingle-substrate addition. Alternatively, R¹ and R² can be configuredwith linker end groups for chemical coupling or with no linker groupsfor a hybridization only protocol. R¹ and R² can be of the general typeXR, wherein X is a linking group and R is a functional group.

The tilde (˜) in FIGS. 10A and 10B denotes a selectively cleavable bondseparating two moieties of the probe member. The end linkages of thetether are attached to the two moieties of the probe member that areseparated by the selectively cleavable bond. The tether links the firstprobe moiety to the second probe moiety, forming a loop bridging theselectively cleavable bond. When the probe member is intact (uncleaved),the probe member can bind with high-fidelity to the template sequenceand the tether is looped in the “constrained configuration”. When thisbond is cleaved, the tether loop can open and the tether is in the“expanded configuration”.

Substrate constructs are reagents used for template-dependent assemblyof a daughter strand, which is an intermediate composition for producingXpandomers. FIG. 10B shows the duplex daughter strand, ahetero-copolymer with repeating subunits, (shown in brackets). Shown arethe daughter strand primary backbone (—P¹˜P²—) and the target templatestrand (—P¹′—P²′—) as a duplex (95). Each subunit of the daughter strandis a repeating motif composed of a probe member and a tether member, T(99), the tether member in constrained configuration. The motifs havespecies-specific variability, indicated here by the “α” superscript.Each particular subunit in the daughter strand is selected from alibrary of motifs by a template-directed process and its probe binds toa corresponding sequence of complementary nucleotides on the templatestrand. In this way, the sequence of nucleobase residues of the probesforms a contiguous, complementary copy of the target template strand.

The daughter strand is composed of an Xpandomer precursor called the“constrained Xpandomer” which is further composed of tethers in the“constrained configuration”. When the tethers (99) convert to their“expanded configuration”, the constrained Xpandomer converts to theXpandomer product.

The daughter strand can be seen to have two backbones, a “primarybackbone”, and the backbone of the “constrained Xpandomer. The primarybackbone is composed of contiguously abutted probe substrates. The“constrained Xpandomer backbone” bypasses the selectively cleavablelinkage between probe moieties P¹ and P² and is formed by linkedbackbone moieties, each backbone moiety being a linear linkage of P¹ tothe tether to P², and where P² can further link to the P¹ of nextbackbone moiety. It can be seen that the constrained Xpandomer backbonebridges or loops over the selectively cleavable bonds of the primarybackbone, and will remain covalently intact when these selectivelycleavable bonds are cleaved and the primary backbone is fragmented.

FIG. 10C is a representation of the Class I Xpandomer product afterdissociation of the template strand and after cleavage of theselectively cleavable bonds of the primary backbone. Methods fordissociation of the template strand include heat denaturation, orselective digestion with a nuclease, or chemical degradation. TheXpandomer product strand contains a plurality of subunits κ, where κdenotes the κ^(th) subunit in a chain of m subunits making up thedaughter strand, where κ=1, 2, 3 to m, where m>3, and generally m>20,and preferably m>50, and more preferentially m>1000. Each subunit isformed of a tether (99) and probe moieties P¹ and P². The tether memberT, now in “expanded configuration”, is seen stretched to its lengthbetween the cleaved probe moieties P¹ and P², which remain covalentlylinked to the adjacent subunits. Each subunit, a subunit motif α,contains species-specific genetic information established by templatedirected assembly of the Xpandomer intermediate (daughter strand).

FIG. 10D shows the substrate construct of FIG. 10A as a molecular model,where the probe member (100) is arbitrarily represented with fournucleobase residues (101,102,103,104), two of which (102,103) are joinedto the tether (99) by end linkages (108,109). Between the two endlinkages of the tether is a selectively cleavable bond, shown as the “V”(110) in probe member (100). This bond joins probe moieties P¹ and P²referred to in FIG. 10A. The tether loop shown here has three reporters(105,106,107), which can also be motif species specific.

FIG. 10E shows the product Xpandomer after cleavage of the selectivelycleavable bonds in the substrate. Cleavage results in expansion of theconstrained Xpandomer and is denoted by “E” (dark arrows). The residues(110 a,110 b) of the selectively cleavable bond mark the cleavage event.The subunit is indicated by dotted lines vertically bracketing therepeating subunit, as represented by brackets in the accompanying FIG.10C.

In the Xpandomer product (FIG. 10E) the primary backbone is nowfragmented and not covalently intact because the probe members have beencleaved, separating each P¹ (92) and P² (94). Through the cleavageprocess, the constrained Xpandomer is released to become the Xpandomerproduct. The Xpandomer includes each concatenated subunit in sequence.Linked within each subunit are the probe moiety P¹, the tether, andprobe moiety P². The tether members (99) of the Xpandomer, which wereformerly in constrained configuration, are now in expandedconfiguration, thereby functioning to linearly stretch out the sequenceinformation of the template target. Expanding the tethers lowers thelinear density of the sequence information along the Xpandomer andprovides a platform for increasing the size and abundance of reporterswhich in turn improves signal to noise for detection and decoding of thetemplate sequence.

FIG. 11 depicts a condensed schematic of a method for making a anembodiment of a Class I Xpandomer; the method illustrates the making andusing of substrates and products shown in FIGS. 10D and 10E. The methodmay be performed in free solution and is described using a ligase (L) tocovalently couple abutting Xprobes. Methods for relieving secondarystructure in the template are discussed in a subsequent section.Conditions adapted for hybridization and ligation are well known in theart and such conditions can be readily optimized by one of ordinaryskill in this field.

Ligases include, but are not limited to, NAD⁺-dependent ligasesincluding tRNA ligase, Taq DNA ligase, Thermus filiformis DNA ligase,Escherichia coli DNA ligase, Tth DNA ligase, Thermus scotoductus DNAligase, thermostable ligase, Ampligase thermostable DNA ligase,VanC-type ligase, 9°N DNA Ligase, Tsp DNA ligase, and novel ligasesdiscovered by bioprospecting. Ligases also include, but are not limitedto, ATP-dependent ligases including T4 RNA ligase, T4 DNA ligase, T7 DNAligase, Pfu DNA ligase, DNA ligase 1, DNA ligase III, DNA ligase IV, andnovel ligases discovered by bioprospecting. These ligases includewild-type, mutant isoforms, and genetically engineered variants.

Referring to FIG. 11, and in preparation for the synthesis, a targetnucleic acid (110) is provided and the ends are polished in preparationfor blunt-ended ligation of adaptors. Step I shows the ligation ofhairpin primers (120) to the target nucleic acid. The free 5′ end of theprimers are blocked with a removable blocking group (119). The primerswill prime both strands of the target nucleic acid. The adaptors aregenerally added in excess. The blocking groups on the hot ends of theprimers are removed in Step II, and the two strands of the template areseparated by denaturation. In Step III, the primed single-strandedtemplate (111) is contacted with a substrate construct library (asrepresented by construct (112) for purpose of illustration) and withligase, L, under conditions permissible for hybridization ofcomplementary probe substrate (113) and ligation at the reactive end ofthe primer, as is shown in Step IV. Generally, hybridization andligation is performed at a temperature greater than the meltingtemperature of the substrate to reduce non-specific side reactions. Eachsubstrate construct in this example contains a tether arrayed with threereporters. Each probe substrate has a selectively cleavable bond(indicated with a “V”) between the two tether attachment sites. In StepV, a second substrate construct (114) is added by template-directedhybridization and ligation, and so forth. In Step VI, formation of afully extended Xpandomer intermediate (117) is demonstrated. Thisintermediate can be denatured from the template strand and selectivelycleaved at the cleavage sites shown, thereby forming a product Xpandomersuitable for sequencing. In some embodiments, denaturation is not neededand the template strand can be digested in place.

FIG. 12 is a condensed schematic of a second method, here for makinganother embodiment of a Class I Xpandomer. In preparation for thesynthesis, a target nucleic acid (120) is provided and the ends arepolished in preparation for blunt-ended ligation of adaptors (121,122).Step I shows the ligation of doubly-blocked hairpin primer precursors tothe target nucleic acid. One end of the duplex hairpin primers isblocked with removable blocking groups (125 a,125 b,125 c,125 d)intended to prevent ligation and concatenation of the template strandsor adaptors. The adaptors are generally added in excess. The blockinggroups are removed in Step II, and the two strands of the template areseparated by denaturation. In Step III, the hairpin primers self-anneal,forming priming sites (126,127) for the subsequent ligation of substrateconstructs, which can proceed bi-directionally, i.e., both in a ‘3 to 5’and a 5′ to 3′ direction. In Step IV, the primed templates are contactedwith a substrate construct library (128) under conditions permissiblefor hybridization of complementary probe substrates and ligation.Ligation proceeds incrementally (i.e., extending the growing ends withapparent processivity) by a process of hybridization of complementaryprobe substrates and ligation at the ends of the nascent daughterstrands. Each substrate construct in this example contains a tether looparrayed with reporter groups. In Step V, formation of a completedXpandomer intermediate (129) is depicted. Optionally the template strandcan be removed by nuclease digestion, freeing the Xpandomer. Theintermediate can be selectively cleaved at the cleavage sites shown,thereby forming a product Xpandomer suitable for sequencing. The productXpandomers are formed in free solution.

In FIG. 13, a method relying on immobilized template strands is shown.Here the template strands are anchored to a bead (or other solid phasesupport) by an adaptor (131). The template is shown in contact withsubstrate constructs (132), and in Step I, the conditions are adapted sothat hybridization occurs. It can be seen that “islands” of hybridized,abutting substrate constructs are formed. In Step II, addition ofligase, L, results in ligation of the abutting substrate constructs,thereby forming multiple contiguous sequences of ligated intermediatesseparated by gaps. In Step III, conditions are adjusted to favordissociation of low molecular weight or mismatched hybridized material,and in Step IV, the reactions of Steps II through III are repeated oneor more times to favor formation of longer extension products. Thisprimerless process is referred to herein as “promiscuous ligation”.Ligation can extend bidirectionally and nicked junctions can be sealedwith ligase, thereby filling gaps. In Step V, after optimization of thedesired product lengths, the immobilized duplexes are washed to removeunreacted substrate and ligase. Then, in Step VI, the daughter strands(here shown as a single-stranded Xpandomer intermediate) (138,139) aredissociated from the template. Selective cleavage of selectivelycleavable bonds of the intermediate results in formation of theXpandomer product (not shown). In this embodiment, the immobilizedtemplate can be reused. Once the Xpandomer products are sequenced,contigs can be assembled by well known algorithms for overlapping andaligning the data to build a consensus sequence.

Referring to FIG. 14, a method for using immobilized primers is shown.End-adapted (or random template sequences, depending on the nature ofthe immobilized primers) templates (142), are annealed to theimmobilized primers (140) in Step I. In Step II, the immobilizedtemplates (143) are contacted with a substrate construct library,members of which are shown as (144), and conditions are adjusted fortemplate-directed hybridization. In this example, the substrateconstruct 3′ OH termini of the probe members (R group) has beensubstituted (146) to reversibly block further extension. In Step III,the abutting ends of the adjacent substrate construct and primer, orfree end of the nascent growing daughter strand, are ligated and the 3′OH end of the nascent daughter strand is activated by removing theblocking R group (146). As indicated in Steps IV and V, this process ofstepwise cyclical addition can be repeated multiple times. Typically awash step is used to remove unreacted substrates between each extensionstep.

The process is thus analogous to what is termed “cyclical single baseextension”, but would more properly be termed here “cyclical singleprobe extension”. While ligase, L, is shown, the process can beperformed with a ligase, polymerase, or by any chemical couplingprotocol suitable for joining oligomers in a template-directedsynthesis. The chemical coupling may occur spontaneously at the abuttingends of the hybridized probes, or a condensing agent may be added at thebeginning of Step III and each ensuing Step V of the cycle. Theterminally blocking R group is configured so that free run-onpolymerization cannot occur on the template or in solution. Step VIshows the formation of a complete Xpandomer intermediate (149); no moresubstrate can be added. This intermediate can be dissociated from thetemplate, the single-stranded product is then cleaved to open up thebackbone as previously described.

This method can be adapted for selective sequencing of particulartargets in a nucleic acid mixture, and for parsed sequencing methods onsequencing arrays, for example, by non-random selection of theimmobilized primers. Alternatively, universal or random primers may beused as shown.

FIG. 15 describes a method for promiscuous hybridization on animmobilized template (150) (Step I), where the substrate constructs ofthe library (152) are modified with a chemical functional group that isselectively reactive (156), depicted as an open triangle, with anabutting probe. A detail of the chemical functional group of thesubstrate constructs is shown in the expanded portion shown by thehatched circle (FIG. 15 a). At a certain density of hybridization,coupling is initiated as shown in Step II, resulting in high molecularweight Xpandomer intermediates linked by the crosslinked product (157),depicted as a filled triangle, of the coupling reaction. A detail of thecrosslinked probes is shown in the expanded portion shown in the hatchedcircle (FIG. 15 b) in the product of Step II. This process can beaccompanied by steps for selective dissociation and removal of lowmolecular weight products and any possible mismatched products. Couplingchemistries for this method of promiscuous chemical coupling are knownto someone skilled in the art and include, for example, the techniquesdisclosed in U.S. Pat. No. 6,951,720 to Burgin et al.

In another embodiment, polymerase-based methods are disclosed forassembling product Xpandomers. Generally, substrate triphosphates(Xmers) are the appropriate substrate for reactions involving apolymerase. The selection of a suitable polymerase is part of a processof optimizing the experimental protocol. As shown in FIG. 16 forillustration, and while not intended to be limiting, a reaction mixturethat contains a template (160) and a primer (161) is contacted with alibrary of substrate constructs (162) and a polymerase (P), underconditions optimized for template-directed polymerization. In Step I,the polymerase begins to processively add dinucleotide Xmers (tetherswith two reporters) to the template strand. This process continues inSteps II and III. Each probe subunit added is a particular speciesselected by specific binding to the next adjacent oligomer of thetemplate so as to form a contiguous complementary copy of the template.While not bound by theory, the polymerase is thought to assist inensuring that incoming probe species added to the nascent chain arespecifically complementary to the next available contiguous segment ofthe template. Loeb and Patel describe mutant DNA polymerases withincreased activity and improved fidelity (U.S. Pat. No. 6,329,178).Williams for example, in U.S. Patent Application 2007/0048748 has shownthat polymerases can be modified for increased speed of incorporationand reduction in error rate, clearly linking error rate not withhybridization accuracy but rather with polymerase processivity. Step IIIresults in a completed Xpandomer intermediate (168). The single-strandedXpandomer intermediate is then treated by a process that can involvedenaturation of the template strand (not shown). The primary backbone ofthe daughter strand is selectively cleaved to expand the tethers,thereby forming an Xpandomer product suitable for use in a sequencingprotocol, as previously explained.

As shown in FIG. 17, polymerase driven template-directed synthesis of anXpandomer can be achieved by alternative techniques. Here, animmobilized primer (170) to which a processed template strand (171) isannealed in Step I. In Step II, polymerase, P, processively couplesspecifically complementary substrate constructs (175) from a library ofsuch constructs (depicted by 174) in the reaction mixture. Conditionsand reagent solutions are adjusted to favor processive polymeraseactivity. As shown here, hybridization in Step II and polymerization inStep III are separate activities, but the activities of the polymeraseneed not be isolated in that way. In Step IV, incremental processiveaddition of complementary substrate constructs continues cyclically(continuously without interruption), resulting in the fully loadedXpandomer intermediate (177) as depicted resulting from Step IV. TheXpandomer intermediate can be dissociated and expanded in preparationfor its use in a sequencing protocol as previously described. Note thatthis method also lends itself to parsed sequencing methods by selectionof suitable immobilized primers. Further, methods for stretching thetemplate to relief secondary structure are readily adapted to thismethod, and is discussed in subsequent sections.

Polymerases include, but are to limited to, DNA-dependent DNApolymerases, DNA-dependent RNA polymerases, RNA-dependent DNApolymerases, RNA-dependent RNA polymerases, T7 DNA polymerase, T3 DNApolymerase, T4 DNA polymerase, T7 RNA polymerase, T3 RNA polymerase, SP6RNA polymerase, DNA polymerase 1, Klenow fragment, Thermophilusaquaticus DNA polymerase, Tth DNA polymerase, VentR® DNA polymerase (NewEngland Biolabs), Deep VentR® DNA polymerase (New England Biolabs), BstDNA Polymerase Large Fragment, Stoeffel Fragment, 9°N DNA Polymerase,9°N DNA polymerase, Pfu DNA Polymerase, Tfl DNA Polymerase, Tth DNAPolymerase, RepliPHI Phi29 Polymerase, Tli DNA polymerase, eukaryoticDNA polymerase beta, telomerase, Therminator™ polymerase (New EnglandBiolabs), KOD HiFi™ DNA polymerase (Novagen), KOD1 DNA polymerase,Q-beta replicase, terminal transferase, AMV reverse transcriptase, M-MLVreverse transcriptase, Phi6 reverse transcriptase, HIV-1 reversetranscriptase, novel polymerases discovered by bioprospecting, andpolymerases cited in US 2007/0048748, U.S. Pat. No. 6,329,178, U.S. Pat.No. 6,602,695, and U.S. Pat. No. 6,395,524 (incorporated by reference).These polymerases include wild-type, mutant isoforms, and geneticallyengineered variants.

Class II and III Oligomeric Constructs

Referring to FIGS. 18A through 18E, describe Class II oligomericconstructs in more detail, which (along with the isomeric Class IIIoligomeric constructs) can be either Xprobes or Xmers.

FIGS. 18A through 18C are read from left to right, showing first theprobe substrate construct (oligomeric precursor of Xpandomer), then theintermediate duplex daughter strand in the center, and on the right theXpandomer product prepared for sequencing.

As shown in FIG. 18A, a Class II substrate construct has an oligomericprobe member (—P¹—P²—) (180) and a tether member, T (181). The tether isattached by a single end linkage (184) of a first end moiety to probemoiety P². At the distal end of the tether (186), a second end moietyhas a linker group δ and is positioned proximate to R². The second endmoiety also has a cleavable intra-tether crosslink (187) to constrain itto this location. Cleavable crosslink (187) is denoted by a dotted line,which can indicate, for example, a disulfide bond. These constraintsprevent the tether from elongating or expanding and thus is in aconstrained configuration. A second linker group is positioned near thedistal end (189) of the probe member near R¹. Under template-directedassembly, substrates form a duplex with the target template such thatthe substrates are abutted. Under controlled conditions, linker groups δand ε of the abutting substrates link to form a χ-bond between theadjacent substrate constructs (shown in FIGS. 18B and 18C). Theselinkage groups are positioned on the substrate construct to limit theselinkage reactions to adjacent abutted substrate constructs. Thesubstrate construct preferentially does not link with itself. Suitablelinkage and protection/deprotection chemistries for δ, ε, and χ aredetailed in the general oligomeric construct description.

R¹ and R² are end groups configured as appropriate for the synthesisprotocol in which the substrate construct is used. For example,R¹=5′-phosphate and R²=3′-OH, would find use in a ligation protocol, andR¹=5′-triphosphate and R²=3′-OH for a polymerase protocol. Optionally,R² can be configured with a reversible blocking group for cyclicalsingle-substrate addition. Alternatively, R¹ and R² can be configuredwith linker end groups for chemical coupling or with no linker groupsfor a hybridization only protocol. R¹ and R² can be of the general typeXR, wherein X is a linking group and R is a functional group.

Substrate constructs are reagents used for template-dependent assemblyof a daughter strand, an intermediate composition for producingXpandomers. FIG. 18B shows the duplex daughter strand, ahetero-copolymer with repeating subunits (shown in brackets). Shown aredaughter strand primary backbone (˜P¹—P²˜) and target template strand(—P¹—P²—) as a duplex (185). Each subunit of the daughter strand is arepeating motif comprising a probe member and a tether member. Themotifs have species-specific variability, indicated here by the asuperscript. Each particular subunit in the daughter strand is selectedfrom a library of motifs by a template-directed process and its probebinds to a corresponding sequence of complementary nucleotides on thetemplate strand. In this way, the sequence of nucleobase residues of theprobes forms a contiguous, complementary copy of the target templatestrand.

Each tilde (˜) denotes a selectively cleavable bond. The internal bondbetween moieties P¹ and P² of a probe member are not selectivelycleavable bonds but the inter-probe bonds (between subunits) arenecessarily selectively cleavable as required to expand the tethers andthe Xpandomer. In one embodiment, no direct bond is formed between theprobes of separate subunits, thereby eliminating the need for subsequentselective cleavage.

The daughter strand is composed of the precursor Xpandomer called the“constrained Xpandomer” which is further composed of tethers in the“constrained configuration”. When the tethers convert to their “expandedconfiguration”, the constrained Xpandomer converts to the Xpandomerproduct. The tethers are constrained by the χ linkages formed bybridging to the probe members of adjacent subunits and, optionally, theintratether linkages if still present. The χ linkage attaches the tethermember of a first subunit to the abutting end of an adjacent secondsubunit and is formed by linking the collocated linker groups δ, of thefirst subunit, and ε, of the second subunit.

The daughter strand can be seen to have two backbones, a “primarybackbone”, and the backbone of the “constrained Xpandomer. The primarybackbone is composed of the contiguously abutted probe substrates. The“constrained Xpandomer backbone” bypasses the selectively cleavablelinkage between the subunit substrates and is formed by χ bond linkedbackbone moieties, each backbone moiety being a linear linkage oftether, to P², to P¹, each χ bond linking P¹ to the tether of the nextbackbone moiety. It can be seen that the constrained Xpandomer backbonebridges or loops over the selectively cleavable bonds of the primarybackbone, and will remain covalently intact when these selectivelycleavable bonds are cleaved and the primary backbone is fragmented.

In FIG. 18B, the linker groups δ and ε have crosslinked and now form aninter-subunit bond χ. After the χ bond is formed the intra-tether bondmay be broken, although it is shown here intact (dotted line in thesubstrate). Generally, the formation of the χ bond is dependent onproximity of the linker group δ on the first subunit and the position ofthe linker group ε of a second abutting subunit, so that they arecollocated and are contacted during or after template-directed assemblyof substrate constructs.

In further embodiments, the crosslinking is dependent only onhybridization to the template to bring the two linker groups together.In still other embodiments, the χ bond linkage is preceded by enzymaticcoupling of the probe members P along the primary backbone, withformation of phosphodiester bonds between adjacent probes. In thestructure shown here, the daughter strand primary backbone has beenformed, and the inter-substrate bonds are depicted by a tilde (˜) toindicate that they are selectively cleavable. After dissociating ordegrading the target template strand, cleaving the selectively cleavablebonds (which include the intratether bonds), the constrained Xpandomeris released and becomes the Xpandomer product.

FIG. 18C is a representation of the Class II Xpandomer product afterdissociation of the template strand and after cleavage of theselectively cleavable bonds (including those in the primary backboneand, if not already cleaved, the intratether links). Methods fordissociation of the template strand include heat denaturation, orselective digestion with a nuclease, or chemical degradation. TheXpandomer product strand contains a plurality of subunits K, where κdenotes the κ^(th) subunit in a chain of m subunits making up thedaughter strand, where κ=1, 2, 3 to m, where m>3, and generally m>20,and preferably m>50, and more preferentially m>1000. Each subunit isformed of a tether, and probe moieties P¹ and P². Tether, T (181), isseen in its expanded configuration and is stretched to its lengthbetween P² and P¹ of adjacent subunits. Each subunit, a subunit motif α,contains species-specific genetic information established by templatedirected assembly of the Xpandomer intermediate (daughter strand).

FIG. 18D shows the substrate construct of FIG. 18A as a molecular model,where the probe member (180), represented with four nucleobase residues(81,82,83,84), is joined to the tether (181) by a linkage of the firstend moiety of the tether (184). An intra-tether bond (85) of a secondend moiety is at the distal end of the tether. A linker group (δ) (86)is also disposed on the second end moiety and the corresponding secondlinker group (ε) (87) is anchored to the end of the probe opposing thelinker group (δ). The tether loop shown here has three reporters(78,79,80), which can also be motif species specific.

FIG. 18E shows the substrate construct after incorporation into theproduct Xpandomer. The subunits are cleaved and expanded and are linkedby χ bonds (88), formed by linking the linker groups δ and ε referred toin FIG. 18A. A subunit is indicated by dotted lines verticallybracketing the repeating subunit, as represented by brackets in theaccompanying FIG. 18C. “E” again denotes expansion.

In the Xpandomer product (FIG. 18E) the primary backbone has beenfragmented and is not covalently contiguous because any direct bondbetween the probes of adjacent subunits has been cleaved. Through thecleavage process, the constrained Xpandomer is released to become theXpandomer product. The tether members that were formerly in constrainedconfiguration, are now in expanded configuration, thereby functioning tolinearly stretch out the sequence information of the template target.Expanding the tethers lowers the linear density of the sequenceinformation along the Xpandomer and provides a platform for increasingthe size and abundance of reporters which in turn improves signal tonoise for detection and decoding of the template sequence.

While the tether is depicted as a reporter construct with three reportergroups, various reporter configurations can be arrayed on the tether,and can comprise single reporters that identify probe constituents,single reporters that identify probe species, molecular barcodes thatidentify the probe species, or the tether can be a naked polymer. Insome cases, one or more reporter precursors are arrayed on the tether,and reporters are affinity bound or covalently bound following assemblyof the Xpandomer product.

Class III oligomeric constructs, illustrated in FIGS. 19A through 19E,are isomers to the Class II constructs discussed above. No furtherdescription is included because the description of Class II is adequateto understand this class.

This class can serve to emphasize that all classes can be reflected tomirror image application (i.e., exchanging the R¹ and R² groups).Furthermore this serves to illustrate that the classes described are notintended to be complete but to reflect a few of the many possiblearrangements that this invention encompasses.

FIG. 20 depicts a condensed schematic of a method for making a firstembodiment of a Class II Xpandomer; the method illustrates the makingand using of substrates and products shown in FIGS. 18D and 18E. Themethod is performed with solid-phase chemistries. Methods for relievingsecondary structure in the template are discussed in a subsequentsection. Suitable conditions adapted for hybridization and chemicalcoupling are well known in the art and the conditions can be readilyoptimized by one skilled in this field.

Step I of FIG. 20 shows a reaction mixture containing an immobilizedtemplate (200) and a library of substrate reagents (201). The substrateconstructs are seen to specifically bind to the template in atemplate-directed hybridization. Conditions are adjusted to optimize thecomplementarity and fidelity of the binding. As shown in the figureinsert (see FIG. 20 a), each abutting substrate construct brings intoproximity the functional group δ (202) on the distal aspect of thetether, shown here bound to the stem of the tether by an intra-tethercrosslink (203), represented by the adjacent triangles, and thefunctional group ε (204) of the abutting probe member.

In Step II, a crosslinking reaction occurs between hybridizedproximately abutted ends of the probe members involving the twofunctional groups δ and ε, thereby forming an inter-subunittether-to-probe bond χ (205), depicted as an open oval, as shown in thefigure insert (see FIG. 20 b). Hybridization occurs in parallel atvarious sites on the template, promiscuously, and chemical coupling canoccur in a cycle of hybridization (Step III), stringent melt and/or wash(Step IV), and chemical coupling (Step V). The cycle can be repeated toincrease the number of contiguous subunits assembled to form theXpandomer intermediate. Step VI illustrates a completed Xpandomerintermediate with two contiguous product strands of varying length. Asimilar method may be employed with the Class III Xpandomers.

FIG. 21 illustrates a method of processive ligation of Class IIsubstrates on an immobilized template. Step I shows a primer (210)annealing to the template (212), the primer adapted with a chemicallyreactive functional group ε shown in the figure insert as (214) (seeFIG. 21 a). A reaction mixture containing Class II substrates (216) isthen added in Step II. As shown in the figure insert (FIG. 21 a), thesesubstrate constructs have δ (217) and ε (214) reactivity on opposingends of the probe-tether member. A first substrate construct is seen tospecifically bind to the template in a template-directed hybridization.Conditions are adjusted to optimize the complementarity and fidelity ofthe binding. A ligase is then used to covalently bond the first probe tothe primer (Step II).

In Steps III and IV, the process of processive hybridization andligation of substrate constructs is continued in order to build up theXpandomer intermediate shown formed in Step IV. Following this, in StepV, crosslinking is performed between the δ (217) and ε (214) groups (seeFIG. 21 b), resulting in a χ bond as depicted in FIG. 21 c as (219). Asshown in the figure inserts (FIGS. 21 b and 21 c), functional group δ(217) on the tether is constrained by an intra-tether crosslink (211),represented by the adjacent triangles, until the χ bond is formed. Thecompleted Xpandomer intermediate is optionally dissociated from thetemplate strand and is cleaved to form an Xpandomer product suitable forsequencing. A similar method may be employed with the Class IIIXpandomers. This method is can also be adapted for use with a polymeraseby substituting triphosphate substrate constructs.

Class IV and V Oligomeric Constructs

Referring to FIGS. 22A through 22E, describe Class IV oligomericconstructs in more detail.

FIGS. 22A through 22C are read from left to right, showing first theprobe substrate construct (Xprobe or Xmer precursors of Xpandomer), thenthe intermediate duplex daughter strand in the center, and on the rightthe Xpandomer product prepared for sequencing.

FIG. 22A, shows a Class IV substrate construct having oligomeric probemember (229) with probe moieties P¹ and P² attaching to tether, T (220).Tether T is attached to P¹ and P² by appropriate linkage with the firstand second end moieties of the tether, respectively. Linker groups ε ofthe first end moiety and δ of the second end moiety are positioned nearthe R¹ and R² ends of the probe, respectively (in an alternativeembodiment, the positions of the functional groups can be reversed).Under controlled conditions, functional groups δ (222) and ε (221) willreact to form a linkage X as shown in FIG. 22B. These linkage groups arepositioned on the substrate construct to limit these linkage reactionsto adjacent abutted substrate constructs. The substrate constructpreferentially does not link with itself. Suitable linkage andprotection/deprotection chemistries for δ, ε, and χ are detailed in thegeneral oligomeric construct description.

R¹ and R² are end groups configured as appropriate for the synthesisprotocol in which the substrate construct is used. For example,R¹=5′-phosphate and R²=3′-OH, would find use in a ligation protocol, andR¹=5′-triphosphate and R²=3′-OH for a polymerase protocol. Optionally,R² can be configured with a reversible blocking group for cyclicalsingle-substrate addition. Alternatively, R¹ and R² can be configuredwith linker end groups for chemical coupling or with no linker groupsfor a hybridization only protocol. R¹ and R² can be of the general typeXR, wherein X is a linking group and R is a functional group.

Substrate constructs are reagents used for template-directed assembly ofa daughter strand, an intermediate composition for producing Xpandomers.FIG. 22B shows the duplex daughter strand, a hetero-copolymer withrepeating subunits (shown in brackets). Shown are daughter strandprimary backbone (—P¹˜P²—) and target template strand (—P^(1′)—P^(2′)—)as a duplex (228). Each subunit of the daughter strand is a repeatingmotif comprising a probe member and a tether member. The motifs havespecies-specific variability, indicated here by the a superscript. Eachparticular subunit in the daughter strand is selected from a library ofmotifs by a template-directed process and its probe binds to acorresponding sequence of complementary nucleotides on the templatestrand. In this way, the sequence of nucleobase residues of the probesforms a contiguous, complementary copy of the target template strand.

The tilde (˜) denotes a selectively cleavable bond. The internal bondbetween moieties P¹ and P² of a probe member is necessarily selectivelycleavable as required to expand the tethers and the Xpandomer. In oneembodiment, no direct bond is formed between the probes of separatesubunits.

The daughter strand is composed of an Xpandomer precursor called the“constrained Xpandomer” which is further composed of tethers in the“constrained configuration”. When the tethers convert to their “expandedconfiguration”, as shown in FIG. 22C, the constrained Xpandomer convertsto the Xpandomer product. The tethers are constrained by the χ linkagesformed by bridging to the tether member of adjacent subunits and by theprobe linkages. The χ linkage attaches the tether member of a firstsubunit to the tether of an adjacent second subunit and is formed bylinking the collocated linker groups, δ of the first subunit, and ε ofthe second subunit.

The daughter strand can be seen to have two backbones, a “primarybackbone”, and the backbone of the “constrained Xpandomer backbone”. Theprimary backbone is composed of the contiguously abutted probesubstrates. The “constrained Xpandomer backbone” is the linear linkageof the tethers in each subunit linked together by the χ linkagesbypassing the subunit probe substrates. The χ linkage results from areaction of the functional group ε of a first subunit with thefunctional group δ of an abutted second subunit. It can be seen that theconstrained Xpandomer backbone bridges or loops over the selectivelycleavable bonds of the primary backbone, and will remain covalentlyintact when these selectively cleavable bonds are cleaved and theprimary backbone is fragmented.

In FIG. 22B, the linker groups δ and ε have crosslinked and now form aninter-subunit bond χ. Generally, the formation of the χ bond isdependent on the collocation of the linker group δ, on the firstsubunit, and the linker group ε, of a second abutting subunit, so thatthey are contacted during or after template-directed assembly ofsubstrate constructs.

In further embodiments, the χ bond crosslinking is dependent only onhybridization to the template to bring the two linker groups together.In still other embodiments, formation of the χ bond is preceded byenzymatic coupling of the probe members P along the primary backbonewith phosphodiester bonds between adjacent probes. In the structureshown in FIG. 22B, the daughter strand primary backbone has been formed,and the bond between probe moieties is depicted by a tilde (˜) toindicate that it is selectively cleavable. After dissociating ordegrading the target template strand, cleaving the selectively cleavablebonds, the constrained Xpandomer is released and becomes the Xpandomerproduct as shown in FIG. 22C.

In this regard, FIG. 22C is a representation of the Class IV Xpandomerproduct after dissociation of the template strand and after cleavage ofthe selectively cleavable bonds of the primary backbone. Methods fordissociation of the template strand include heat denaturation, orselective digestion with a nuclease, or chemical degradation. TheXpandomer product strand contains a plurality of subunits κ, where κdenotes the κ^(th) subunit in a chain of m subunits making up thedaughter strand, where m>3, and generally m>20, and preferably m>50, andmore preferentially m>1000. Each subunit is formed of a tether (220),and pendant probe moieties P¹ and P². Tether, T, is seen in its expandedconfiguration and is stretched to its length between adjacent subunits.Each subunit, a subunit motif α, contains species-specific geneticinformation established by template-directed assembly of the Xpandomerintermediate (daughter strand).

FIG. 22D shows the substrate construct of FIG. 22A as a molecular model,where the probe member, represented with four nucleobase residues (opencircles), is joined to the tether by a linkage of the first end moietyof the tether. The linker group (221), shown as in FIG. 22A, is also ofthe first end moiety of the tether. A linker group (222), shown as δ inFIG. 22A, is disposed on a second end moiety at the distal end of thetether (220). The tether loop shown here has three reporters(800,801,802), which can also be motif species specific. A selectivelycleavable bond, shown as a “V” (225), is located within probe member(229).

FIG. 22E shows the substrate construct after incorporation into theproduct Xpandomer. The subunits are cleaved, shown as lines (225 a,225b), and expanded and are linked by χ bonds (223,224), formed by linkingthe linker groups δ and ε referred to in FIG. 22A. A subunit isindicated by dotted lines vertically bracketing the repeating subunit,as represented by brackets in the accompanying FIG. 22C.

In the Xpandomer product of FIG. 22E, the primary backbone has beenfragmented and is not covalently contiguous because any direct bondbetween the probes of adjacent subunits has been cleaved. Through thecleavage process, the constrained Xpandomer is released to become theXpandomer product. The tether members that were formerly in constrainedconfiguration, are now in expanded configuration, thereby functioning tolinearly stretch out the sequence information of the template target.Expanding the tethers lowers the linear density of the sequenceinformation along the Xpandomer and provides a platform for increasingthe size and abundance of reporters which in turn improves signal tonoise for detection and decoding of the template sequence.

While the tether is depicted as a reporter construct with three reportergroups, various reporter configurations can be arrayed on the tether,and can comprise single reporters that identify probe constituents,single reporters that identify probe species, molecular barcodes thatidentify the probe species, or the tether can be a naked polymer. Insome cases, one or more reporter precursors are arrayed on the tether,and reporters are affinity bound or covalently bound following assemblyof the Xpandomer product.

Class V substrate constructs are similar to the Class IV constructs, theprimary difference being the position of the cleavable linkers. FIGS.23A through 23C are read from left to right, showing first the probesubstrate construct (Xprobe or Xmer precursors of Xpandomer), then theintermediate duplex daughter strand in the center, and on the right theXpandomer product prepared for sequencing.

FIG. 23A illustrates a Class V substrate construct having first andsecond end moieties of tether T (239) attached with two selectivelycleavable end linkages (234, 238) (depicted as two vertical “˜”). Thesecleavable linkages are then attached to first and second probe moieties,P¹ and P², of an oligomeric probe member (235). Linker groups ε (230)and δ (231) of the said first and second end moieties are positionednear the R¹ and R² ends of the probe (again, the positions of thesefunctional groups can be reversed). Under controlled conditions,functional groups δ and ε are reacted to form a linkage χ. These linkagegroups are positioned on the substrate construct to limit these linkagereactions to adjacent abutted substrate constructs. The substrateconstruct preferentially does not link with itself. Suitable linkage andprotection/deprotection chemistries for δ, ε, and χ are detailed in thegeneral oligomeric construct description.

R¹ and R² are end groups configured as appropriate for the synthesisprotocol in which the substrate construct is used. For example,R¹=5′-phosphate and R²=3′-OH, would find use in a ligation protocol asfound in Xprobes, and R¹=5′-triphosphate and R²=3′-OH for a polymeraseprotocol as found in Xmers. Optionally, R² can be configured with areversible blocking group for cyclical single-substrate addition.Alternatively, R¹ and R² can be configured with linker end groups forchemical coupling or with no linker groups for a hybridization onlyprotocol. R¹ and R² can be of the general type XR, wherein X is alinking group and R is a functional group.

Substrate constructs are reagents used for template-directed assembly ofa daughter strand, an intermediate composition for producing Xpandomers.FIG. 23B shows the duplex daughter strand, a hetero-copolymer withrepeating subunits (shown in brackets). Shown are daughter strandprimary backbone (—P¹—P²—) and target template strand (—P^(1′)—P^(2′)—)as a duplex (236). Each subunit of the daughter strand is a repeatingmotif comprising a probe member and a tether member. The motifs havespecies-specific variability, indicated here by the a superscript. Eachparticular subunit in the daughter strand is selected from a library ofmotifs by a template-directed process and its probe binds to acorresponding sequence of complementary nucleotides on the templatestrand. In this way, the sequence of nucleobase residues of the probesforms a contiguous, complementary copy of the target template strand.

The tilde (˜) denotes a selectively cleavable bond. The bonds connectmoieties P¹ and P² of a probe member with the tether and are necessarilyselectively cleavable as required to expand the tethers and theXpandomer. In one embodiment, no direct bond is formed between theprobes of separate subunits.

The daughter strand is composed of an Xpandomer precursor called the“constrained Xpandomer” which is further composed of tethers in the“constrained configuration”. When the tethers convert to their “expandedconfiguration”, as shown in FIG. 23C, the constrained Xpandomer convertsto the Xpandomer product. The tethers are constrained by the χ linkagesformed by bridging to the tether members of adjacent subunits and by theselectively cleavable linkages (234,238). The χ linkage attaches thetether member of a first subunit to the tether of an adjacent secondsubunit and is formed by linking the collocated linker groups, δ of thefirst subunit, and of the second subunit.

The daughter strand can be seen to have two backbones, a “primarybackbone”, and the backbone of the “constrained Xpandomer backbone”. Theprimary backbone is composed of the contiguously abutted probesubstrates. The “constrained Xpandomer backbone” is the linear linkageof the tethers in each subunit linked together by the χ linkagesbypassing the subunit probe substrates. The χ linkage results from areaction of the functional group ε of a first subunit with thefunctional group δ of an abutted second subunit. It can be seen that theconstrained Xpandomer backbone bridges or loops over the selectivelycleavable bonds connecting to the primary backbone, and will remaincovalently intact when these selectively cleavable bonds are cleaved andthe primary backbone is dissociated or otherwise fragmented.

In FIG. 23B, the linker groups δ and ε have crosslinked and now form aninter-subunit bond χ. Generally, the formation of the χ bond isdependent on the collocation of the linker group δ, on the firstsubunit, and the linker group ε, of a second abutting subunit, so thatthey are contacted during or after template-directed assembly ofsubstrate constructs.

In some protocols, the crosslinking reaction is dependent only onhybridization to the template to bring the two reactive groups together.In other protocols, the linking is preceded by enzymatic coupling of theprobe members, with formation of phosphodiester bonds between adjacentprobes. In the structure shown in FIG. 23B, the daughter strand primarybackbone has been formed. The tether, now joined to adjacent subunits byχ-bonds, and comprises the constrained Xpandomer backbone. Upon cleavageof the selectively cleavable bonds (˜), the constrained Xpandomer isseparated from the primary backbone to become the Xpandomer product, andits now unconstrained tethers are linearly expanded to their full lengthas shown in FIG. 23C.

In this regard, FIG. 23C is a representation of the Class V Xpandomerproduct after cleavage of the selectively cleavable bonds thatdissociates the primary backbone. The Xpandomer product strand containsa plurality of subunits κ, where κ denotes the κ^(th) subunit in a chainof m subunits making up the daughter strand, where m>3, and generallym>20, and preferably m>50, and more preferentially m>1000. Each subunitis formed of a tether, T (239), as seen in its expanded configurationand is stretched to its length between adjacent subunits. Each subunit,a subunit motif a, contains species-specific genetic informationestablished by template-directed assembly of the Xpandomer intermediate(daughter strand).

FIG. 23D shows the substrate construct of FIG. 23A as a molecular model,where the probe member (235), represented with four nucleobase residues(open circles), is joined to the first and second end moieties of thetether by two cleavable linkages (234,238). The linker group (232),shown as ε in FIG. 23A, is of the first end moiety of the tether andlinker group (233), shown as δ in FIG. 23A, is of the tether second endmoiety. The tether loop shown here has three reporters (237 a, 237 b,237 c), which can also be motif species specific.

FIG. 23E shows the substrate construct after incorporation into theproduct Xpandomer. The subunits are cleaved (234 a, 234 b, 238 a, 238 b)and expanded and are linked by χ bonds (249,248), formed by linking thelinker groups δ and ε referred to in FIG. 23A. A subunit is indicated bydotted lines vertically bracketing the repeating subunit, as representedby brackets in the accompanying FIG. 23C.

In the Xpandomer product of FIG. 23E, the primary backbone (235) hasbeen cleaved off (dissociated). Through the cleavage process, theconstrained Xpandomer is released to become the Xpandomer product. Thetether members that were formerly in constrained configuration, are nowin expanded configuration, thereby functioning to linearly stretch outthe sequence information of the template target. Expanding the tetherslowers the linear density of the sequence information along theXpandomer and provides a platform for increasing the size and abundanceof reporters which in turn improves signal to noise for detection anddecoding of the template sequence.

While the tether is depicted as a reporter construct with three reportergroups, various reporter configurations can be arrayed on the tether,and can comprise single reporters that identify probe constituents,single reporters that identify probe species, molecular barcodes thatidentify the probe species, or the tether can be a naked polymer. Insome cases, one or more reporter precursors are arrayed on the tether,and reporters are affinity bound or covalently bound following assemblyof the Xpandomer product.

Making and use of a Class V Xpandomer is illustrated in FIG. 24. Insetting up for the synthesis in Step I, a single stranded template (245)is contacted and annealed with sequencing primer (246). The primerassembly (247) is then contacted with a library of Class V substrateconstructs and a polymerase (Step II). In Step III, the substrates havebeen added processively in a template-directed polymerization. In StepIV, polymerization of the primary backbone of the daughter strand iscompleted and the reactive functional groups of abutting tether sidearms are crosslinked, forming the tether-to-tether χ bonds. Finally, inStep V, the cleavable bonds in the stems of the tether loops are cut,releasing the synthetic tether-to-tether backbone from the oligomericdaughter strand and template. This Xpandomer (249) is thus entirelyconstructed of tether linkages and is shown to spontaneously expand asit drifts away from the remainder of the synthetic intermediate. Heregenetic information corresponding to the target polynucleotide sequenceis encoded in the contiguous subunits of the tethers.

Making and Using Xmers

Class I embodiments include Xprobes and Xmers. Xprobes aremonophosphates, while Xmers are triphosphates. “Xmers” are expandableoligonucleotide triphosphate substrate constructs that can bepolymerized in an enzyme-dependent, template-directed synthesis of anXpandomer. Like Xprobes, Xmer substrate constructs have a characteristic“probe-loop” form as illustrated in FIGS. 10A and 10C, where R¹ is5′-triphosphate and R² is 3′-OH. Note that the substrate constructs areoligonucleobase triphosphates or oligomer analog triphosphates, but theprobe members (i.e., the oligomer) have been modified with a tetherconstruct and a selectively cleavable bond between the end linkages ofthe tether as shown in FIG. 10D, the function of which is furtherillustrated in FIG. 10E.

DNA and RNA polymerases can incorporate dinucleotide, trinucleotide, andtetranucleotide triphosphate oligonucleotides with a level of efficiencyand fidelity in a primer-dependent, processive process as disclosed inU.S. Pat. No. 7,060,440 to Kless. Tether modified oligonucleotidetriphosphates of length n (n=2, 3, 4, or more) can be used as substratesfor polymerase-based incorporation into Xpandomers. Suitable enzymes foruse in the methods shown in FIGS. 16 and 17 include, for example,DNA-dependent DNA polymerases, DNA-dependent RNA polymerases,RNA-dependent DNA polymerases, RNA-dependent RNA polymerases, T7 DNApolymerase, T3 DNA polymerase, T4 DNA polymerase, T7 RNA polymerase, T3RNA polymerase, SP6 RNA polymerase, DNA polymerase 1, Klenow fragment,Thermophilus aquaticus DNA polymerase, Tth DNA polymerase, VentR® DNApolymerase (New England Biolabs), Deep VentR® DNA polymerase (NewEngland Biolabs), Bst DNA Polymerase Large Fragment, Stoeffel Fragment,9°N DNA Polymerase, 9°N DNA polymerase, Pfu DNA Polymerase, Tfl DNAPolymerase, Tth DNA Polymerase, RepliPHI Phi29 Polymerase, Tli DNApolymerase, eukaryotic DNA polymerase beta, telomerase, Therminator™polymerase (New England Biolabs), KOD HiFi™ DNA polymerase (Novagen),KOD1 DNA polymerase, Q-beta replicase, terminal transferase, AMV reversetranscriptase, M-MLV reverse transcriptase, Phi6 reverse transcriptase,HIV-1 reverse transcriptase, novel polymerases discovered bybioprospecting, and polymerases cited in US 2007/0048748, U.S. Pat. No.6,329,178, U.S. Pat. No. 6,602,695, and U.S. Pat. No. 6,395,524(incorporated by reference). These polymerases include wild-type, mutantisoforms, and genetically engineered variants.

Xmer polymerization is a method for Xpandomer synthesis and isillustrated in FIG. 16, for example, where the 2mer substrate isprovided as a triphosphate. Because Xmers are polymerized processively,the extension, crosslinking, end activation, and high stringency washingsteps typically associated with cyclical sequencing by synthesis methodsare optionally eliminated with this approach. Thus the reaction can beperformed in solution. Xpandomer synthesis with Xmers can also beperformed with immobilized templates, as illustrated in FIG. 17, where a4mer Xmer triphosphate is processively polymerized in atemplate-directed synthesis dependent on a primer.

A variety of methods can be employed for robust synthesis of 5′triphosphate Xmers. As described by Burgess and Cook (“Syntheses ofNucleoside Triphosphates”, Chem. Rev. 100(6):2047-2060, 2000), thesemethods include (but are not limited to) reactions using nucleosidephosphoramidites, synthesis via nucleophilic attack of pyrophosphate onactivated nucleoside monophosphates, synthesis via nucleophilic attackof phosphate on activated nucleoside pyrophosphate, synthesis vianucleophilic attack of diphosphate on activated phosphate synthon,synthesis involving activated phosphites or phosphoramidites derivedfrom nucleosides, synthesis involving direct displacement of5′-O-leaving groups by triphosphate nucleophiles, and biocatalyticmethods. A representative method for producing polymerase compatibledinucleotide substrates uses N-methylimidazole to activate the 5′monophosphate group; subsequent reaction with pyrophosphate(tributylammonium salt) produces the triphosphate (Abramova et al., “Afacile and effective synthesis of dinucleotide 5′-triphosphates”,Bioorganic and Med Chem 15, 6549-6555, 2007).

As is discussed in more detail below, the Xmer tether construct isrelated in design, composition and linkage to tether's used for Xprobes.In many embodiments, genetic information is encoded on the tether, andtherefore each tether of each substrate construct is a species-specifictether. The information encoded on the tether is coded with a reportercode that digitizes the genetic information. For example, five bit,binary coding on the tethers would produce 32 unique sequence codes(2⁵). This strategy can be used to uniquely code for all 16 combinationsof two nucleobase residues per probe member of a 2mer library,regardless of the tether's orientation. Similar to Xprobe coding, avariety of functionalization and labeling strategies can be consideredfor Xmers, including (but not limited to): functionalized dendrimers,polymers, branched polymers, nanoparticles, and nanocrystals as part ofthe tether scaffold, as well as reporter chemistries and reportersignals—to be detected with the appropriate detection technology.Base-specific labels can be introduced (via attachment to the tether)either prior to or after Xmer polymerization, by covalent or byaffinity-directed binding.

Design and Synthesis of Xprobes and Xmers

An overview of synthetic and cleavage strategies are presented below,beginning with the probe oligomers with selectively cleavable bonds,followed by the tether and reporter tether constructs.

One objective of an Xprobe- or Xmer-based SBX method is to assemble areplica of the target nucleic acid as completely and efficiently aspossible by a template-directed synthesis, generally a process orcombination of processes selected from hybridizing, ligating,polymerizing, or chemically crosslinking of suitable precursorcompositions, termed here “substrates”. Xprobes and Xmers substrates aresupplied as reagent libraries (e.g., as parts of kits for sequencing)for this purpose. The libraries are generally combinatorial in nature,and contain probe members selected to specifically bind to any or all ofthe complementary sequences such as would be found in a targetpolynucleotide. The number of probes required in a library for thispurpose is a function of probe size. Each probe can be considered to bea sequence fragment, and sufficient variety of probe members must bepresent to form a contiguous copy of the contiguous sequence ofcomplementary sequence fragments of the target polynucleotide. Forprobes in which each oligomer is a dimer, 16 possible speciescombinations of A, T, C and G exist. For probes in which each oligomeris a trimer, then 64 possible species combinations of A, T, C and Gexist, and so forth. When sequencing random genomic fragments, it islikely that all such species are required in a reagent library.

Xprobes and Xmers are oligomeric substrate constructs that are dividedinto five different functional classes. Oligomeric substrate constructshave two distinct functional components: a modified oligonucleobase or“probe” member, and a tether member (“T”). The probe is joined to thetether member by a “probe-loop” construction, where the tether loop is aprecursor of the linearized tether member of the final productXpandomer. Each tether T can be encoded with reporters (commonlyreferred to as “tags” or “labels”), or combinations thereof, thatuniquely identify the probe sequence to which it is tethered. In thisway, the sequence information of the assembled Xpandomer is more readilydetected.

The oligomer is the probe portion of the Xprobe. The probe is a modifiedoligonucleobase having a chain of x deoxyribonucleotides,ribonucleotides, or more generally, nucleobase residues (where x can 2,3, 4, 5, 6, or more). In these discussions a probe with 2, 3, 4, 5 or 6nucleobase residues in length can be referred to as a 2mer, 3mer, 4mer,5mer, or 6mer, respectively.

Substrate construct reagents can be synthesized with an oligonucleotide5′-3′ phosphodiester backbone, the oligomer having the nucleotides A, T,G and C (structures shown in the table of FIG. 25), or otherhybridizable nucleic acid analogs such as those having a peptidebackbone, phosphono-peptide backbone, serine backbone, hydroxyprolinebackbone, mixed peptide-phosphono-peptide backbone, mixedpeptide-hydroxyproline backbone, mixed hydroxyproline-phosphono-peptidebackbone, mixed serine-phosphono-peptide backbone, threose backbone,glycol backbone, morpholino-backbone, and the like, as are known in theart. Deoxyribonucleic acid oligomers and ribonucleic acid oligomers, andmixed oligomers of the two, may also be used as probes. Other bases mayalso be substituted, such as uracil for thymidine, and inosine as adegenerate base. Fragmentary residues of nucleobases havingcomplementarity can also be used.

A more complete recitation of degenerate and wobbly bases known in theart includes, but is not limited to, xanthine, hypoxanthine, or aheterocyclic derivative, analog, or tautomer of xanthine andhypoxanthine, 8-azapurine, purines substituted at the 8 position withmethyl- or bromo-, 9-oxo-N⁶-methyladenine, 2-aminoadenine,7-deazaxanthine, 7-deazaguanine, 7-deaza-adenine, N⁴-ethanocytosine,2,6-diaminopurine, N⁶-ethano-2,6-diaminopurine, 5-methylcytosine,5-(C₃-C₆)-alkynylcytosine, 5-fluorouracil, 5-bromouracil, thiouracil,2-hydroxy-5-methyl-4-triazolopyridine, isocytosine, pseudoisocytosine,isoguanine, 7,8-dimethylalloxazine, 6-dihydrothymine, 5,6-dihydrouracil,4-methyl-indole, ethenoadenine and the nucleobases described in U.S.Pat. Nos. 5,432,272 and 6,150,510, published PCTs WO 92/002258, WO93/10820, WO 94/22892, and WO 94/22144, and in Fasman, PracticalHandbook of Biochemistry and Molecular Biology, pp. 385-394, CRC Press,Boca Raton, La., 1989 (each of which are herein incorporated byreference in their entireties).

As is known in the art, oligomers can be designed to include nucleotidemodifiers. In some embodiments, these serve as the attachment points forthe tether member or members. Purine and pyrimidine derivatives suitablefor synthesis of derivatized oligomers are well known in the art. Twosuch representative modified bases are shown in FIGS. 26A and 26B,wherein a 5-amino-modified cytosine derivative and an 8-amino-modifiedguanine residue are depicted.

As illustrated in FIGS. 27A and 27B, taking a 4mer oligomer as anexample (here illustrated as 5′-monophosphate), any two of the four basepositions on the oligomer can be modified to create attachment points byknown chemistries. Modified nucleotides at probe residues 2 and 3 (onopposite sides of a selectively cleavable bond, depicted as “V”) isillustrated in FIG. 27A. This figure illustrates a 4mer oligomer withamino linkers attached to cytosine and guanosine of the oligomer. FIG.27B illustrates a 4mer oligomer with benzaldehyde functional groups tothe cytosine and guanosine of the oligomer. The details are illustrativeof methods well known in the art. For simplicity, most illustrationsprovided herein will assume 4mers unless otherwise noted, but it isunderstood that other substrate construct libraries or librarycombinations may be employed in the practice of this invention.

Cleavage

Generally, Xprobes and Xmer substrate constructs have selectivelycleavable bonds that allow for controlled expansion of the tether. Aspreviously referenced, such selective cleavage may be achieved by anynumber of techniques known to one skilled in the art, including, but notlimited to, phosphorothiolate backbone cleavage with metal cations, acidcleavage of phosphoramidate backbone modifications, selective nucleasecleavage of standard phosphodiester linkages using nuclease resistantphosphorothioate modifications for backbone protection, photocleavage ofnitrobenzyl modified backbone linkers, and reduction of disulfide bonds.

Modification of substrate probes to include selectively cleavable bondsare illustrated in FIGS. 28A through 28D and in FIGS. 29A through 29D.FIG. 28A shows an example of an Xprobe dimer with a ribosyl 2′-OH groupsusceptible to cleavage by Ribonuclease H in a DNA/RNA duplex Xpandomerintermediate. The bond is thus selectively cleavable, provided the othernucleotide(s) in the Xprobe are resistant to RNase cleavage (for example2′-o-methyl pentose, 2′ deoxyribose nucleobases, “locked” LNAnucleobases, and glycol- or peptide-linked nucleobases). These cleavagesites have other usages, for example, a ribonucleotide at thepenultimate 5′-nucleobase and an adaptor provides a cleavable linker inbetween the Xpandomer and an immobilized support.

FIG. 28B shows an Xprobe with phosphodiester bond coupling twonucleotides. In this figure, as well as FIGS. 28A, 28C and 28D, tethersfor bridging the selectively cleavable bond of the probe are indicatedat (282) and (284). This bond is selectively cleavable with Mung BeanNuclease, S1 Nuclease, DNase I, or other DNAases, for example, if otherbonds joining the subunit tethers together are nuclease resistant.Synthesis of a 2mer library, for example, with a standard phosphatelinkage in between the tether attachment points and phosphorothioatelinkage(s) at the nucleotide backbone position(s) that are to remainintact, provides the desired cleavage pattern. FIG. 28C is an Xprobedimer held together by a 3′-phosphorothiolate bond, and in FIG. 28D isheld together by a 5′-phosphorothiolate bond. These bonds areselectively cleavable by chemical attack, for example with iodoethanolas described by Gish et al. (“DNA and RNA sequence determination basedon phosphorothioate chemistry”, Science 240(4858): 1520-1522, 1988) orby cleavage with divalent metal cations as described by Vyle et al.(“Sequence- and strand-specific cleavage in oligodeoxyribonucleotidesand DNA containing 3′-thiothymidine”. Biochemistry 31(11): 3012-8,1992). Other backbone cleavage options include, but are not limited to,UV induced photoredox cleavage (as by adaptation of nitrobenzylphotocleavage groups) as described by Vallone et al. (“Genotyping SNPsusing a UV-photocleavable oligonucleotide in MALDI-TOF MS”, Methods Mol.Bio. 297:169-78, 2005), acid cleavage of phosphoramidate linkages asdescribed by Obika et al. (“Acid-Mediated Cleavage of OligonucleotideP3′→N5′ Phosphoramidates Triggered by Sequence-Specific TriplexFormation”, Nucleosides, Nucleotides and Nucleic Acids 26(8,9): 893-896,2007), and periodate catalyzed cleavage of3′-O-B-D-ribofuranosyl-2′-deoxy backbone modifications as disclosed byNauwelaerts et al. (“Cleavage of DNA without loss of genetic informationby incorporation of a disaccharide nucleoside”, Nucleic Acids Research31(23): 6758-6769, 2003).

As with the Xprobes, cleavage of the poly-Xmer backbone to produce anXpandomer is accomplished in a variety of ways. As shown in FIG. 29A,for example, an Xmer containing an RNase digestible ribonucleotide basecan be selectively cleaved at that position provided the othernucleotide(s) in the Xmer are resistant to RNase cleavage (for example2′-O-methyl pentose and 2′ deoxyribose nucleobases, “locked” LNAnucleobases, and glycol- or peptide-linked nucleobases). For the Xmerdescribed in FIG. 29A, the 5′ base is a standard 2′ hydroxylribonucleotide cytidine and the 3′ base is an RNase resistant 2′deoxyribonucleotide guanine. The Xmer design allows for selective RNasecleavage of the Xmer backbone to expand an Xpandomer. Alternatively, asshown in FIG. 29B, DNase can be used to cleave all non-phosphorothioateprotected backbone linkages. Accordingly, a 2mer library, for example,with a standard phosphate linkage in between the tether attachmentpoints and phosphorothioate linkage(s) at the nucleotide backboneposition(s) that are to remain intact, provides the desired cleavagepattern. FIG. 29C is an Xmer dimer held together by a3′-phosphorothiolate bond, and FIG. 29D is held together by a5′-phosphorothiolate bond. These bonds are selectively cleavable bychemical attack, for example, with iodoethanol or by cleavage withdivalent metal cations as previously referenced. Other backbone cleavageoptions include (but are not limited to) UV induced photoredox cleavage(as by adaptation of nitrobenzyl photocleavage groups) and acid cleavageof phosphoramidate linkages, both of which are cited above in FIG. 28.In FIGS. 29A through 29D, tethers for bridging the selectively cleavablebond of the probe are indicated at (292) and (294).

Turning now to FIG. 30, in a first general embodiment of a scheme forClass I “probe-loop” substrate construct synthesis, two nucleobaseresidues (circles) at the second and third positions on the probe aremodified to create attachment points L1 and L2 for the two ends L1′ andL2′ of the tether. The tether is shown here as pre-assembled separatelyand is joined to the probe member in a synthetic step (arrow).Intra-tether disulfide bonds (depicted by the two triangles) may be usedin the assembly and use of these substrate constructs. Introduction of areducing agent to the Xpandomer product will selectively break thedisulfide bridges that hold the tether together, thus allowing expansionof the Xpandomer backbone. Photocleavable bonds are also useful infolding tethers during assembly with subsequent release and unfoldingupon exposure to light.

In other embodiments, the phosphodiester backbone of the substrate canbe modified to create attachment points for the tether as disclosed byCook et al. (“Oligonucleotides with novel, cationic backbonesubstituents: aminoethylphosphonates”, Nucleic Acids Research 22(24):5416-5424, 1994), Agrawal et al. (“Site specific functionalization ofoligonucleotides for attaching two different reporter groups”, NucleicAcids Research 18(18): 5419-5423, 1990), De Mesmaeker et al., (“Amidebackbone modifications for antisense oligonucleotides carrying potentialintercalating substituents: Influence on the thermodynamic stability ofthe corresponding duplexes with RNA- and DNA-complements”, Bioorganic &Medicinal Chemistry Letters 7(14): 1869-1874, 1997), Shaw et al.(Boranophosphates as mimics of natural phosphodiesters in DNA”, Curr MedChem. 8(10):1147-55, 2001), Cook et al. (U.S. Pat. No. 5,378,825), andAgrawal (“Functionalization of Oligonucleotides with Amino Groups andAttachment of Amino Specific Reporter Groups”, Methods in MolecularBiology Vol. 26, 1994). The nucleobase residues making up the probemember can be substituted with nucleobase analogs to alter Xprobefunctionality. For example, Locked Nucleic Acids (“LNA”) can be used toincrease probe duplex stability. If chemical coupling of Xprobes isintended (instead of enzymatic ligation), probe 5′ and 3′ ends can befurther derivatized to allow for chemical crosslinking.

Design, Composition and Synthesis of Reporter Constructs

In one embodiment, tethers are encoded with “reporter constructs” thatuniquely identify the sequence of nucleobase residues (or “probe” ofXprobes, Xmers, and other oligomer substrates of FIG. 8) or nucleobase(as in XNTPs, RT-NTPs and monomeric substrates of FIG. 9) to which it istethered. Reporters are reporters or combinations of reporters generallyassociated with the tethers which serve to “parse” or “encode” thesequence information inherent in the substrates and inherent in theorder in which the substrates are incorporated into the Xpandomer. Insome embodiments, the tether is only a spacer and the reporters are, orare associated with, the substrate.

FIG. 31 depicts a method for substrate tether assembly similar to thatof FIG. 30 but the pre-assembled tether includes reporter groups (shownas the three rectangular portions of the tether) and is termed a“reporter construct”. Reporter constructs and tethers may be made by avariety of polymer chemistries, and their use and synthesis is discussedin more detail here.

In the practice of this invention, tethers can serve a variety offunctions, for example: (1) as a tether to sequentially link, directlyor indirectly, adjacent tethers along the nucleobase backbone, (2) as aspacer to stretch out or expand so as to form an elongated chain oftethered subunits, referred to as an Xpandomer, upon cleavage of thebackbone, and/or (3) optionally comprises reporter constructs orreporter precursors that encode the nucleobase or oligomeric sequenceinformation of the individual substrate construct to which the tether isassociated.

Reporter constructs are physical manifestations of reporter codes, whichare bioinformational and digital in nature. Reporter codes parse orencode the genetic information associated with the probe or nucleobasesequence fragment to which the reporter construct and tether isattached. The reporter constructs are designed to optimize thedetectability of the reporter code by adjusting spatial separations,abundance, and signal strength of the constituent reporters. Thereporter constructs can incorporate a broad range of signal andstructural elements including, but not limited to, polymers, dendrimers,beads, aptamers, ligands and oligomers. These reporter constructs aremade by a variety of polymer chemistries and are discussed furtherbelow.

In one embodiment, the reporter constructs are attached to the probe ornucleobase by a polymer tether. The tethers can be constructed of one ormore durable, aqueous- or solvent-soluble polymers including, but notlimited to, the following segment or segments: polyethylene glycols,polyglycols, polypyridines, polyisocyanides, polyisocyanates,poly(triarylmethyl)methacrylates, polyaldehydes, polypyrrolinones,polyureas, polyglycol phosphodiesters, polyacrylates, polymethacrylates,polyacrylamides, polyvinyl esters, polystyrenes, polyamides,polyurethanes, polycarbonates, polybutyrates, polybutadienes,polybutyrolactones, polypyrrolidinones, polyvinylphosphonates,polyacetamides, polysaccharides, polyhyaluranates, polyamides,polyimides, polyesters, polyethylenes, polypropylenes, polystyrenes,polycarbonates, polyterephthalates, polysilanes, polyurethanes,polyethers, polyamino acids, polyglycines, polyprolines, N-substitutedpolylysine, polypeptides, side-chain N-substituted peptides,poly-N-substituted glycine, peptoids, side-chain carboxyl-substitutedpeptides, homopeptides, oligonucleotides, ribonucleic acidoligonucleotides, deoxynucleic acid oligonucleotides, oligonucleotidesmodified to prevent Watson-Crick base pairing, oligonucleotide analogs,polycytidylic acid, polyadenylic acid, polyuridylic acid, polythymidine,polyphosphate, polynucleotides, polyribonucleotides, polyethyleneglycol-phosphodiesters, peptide polynucleotide analogues,threosyl-polynucleotide analogues, glycol-polynucleotide analogues,morpholino-polynucleotide analogues, locked nucleotide oligomeranalogues, polypeptide analogues, branched polymers, comb polymers, starpolymers, dendritic polymers, random, gradient and block copolymers,anionic polymers, cationic polymers, polymers forming stem-loops, rigidsegments and flexible segments. Such polymers can be circularized atattachment points on a substrate construct as described in, for example,in FIG. 30 and FIG. 31.

The tether is generally resistant to entanglement or is folded so as tobe compact. Polyethylene glycol (PEG), polyethylene oxide (PEO),methoxypolyethylene glycol (mPEG), and a wide variety of similarlyconstructed PEG derivatives (PEGs) are broadly available polymers thatcan be utilized in the practice of this invention. Modified PEGs areavailable with a variety of bifunctional and heterobifunctional endcrosslinkers and are synthesized in a broad range of lengths. PEGs aregenerally soluble in water, methanol, benzene, dichloromethane, and manycommon organic solvents. PEGs are generally flexible polymers thattypically do not non-specifically interact with biological chemicals.

FIG. 32A illustrates the repeating structure of a PEG polymer. FIG. 32Bshows an Xprobe or Xmer with a naked PEG tether secured to the probebackbone by, for example amine-terminated linkers (not shown) usingstandard linker chemistries. FIG. 32C shows the same substrate constructafter cleavage of the probe backbone at a selectively cleavable bond(“V”), the PEG polymer flexibly accommodating elongation of theXpandomer. In some embodiments, PEG polymer segments are assembledpiecewise on the tether to provide expansion length or to minimizesteric issues, such as, for example, at the stems of the tethersproximate to the terminal end linkages connecting the tether arms to thesubstrate.

Other polymers that may be employed as tethers, and provide“scaffolding” for reporters, include, for example, poly-glycine,poly-proline, poly-hydroxyproline, poly-cysteine, poly-serine,poly-aspartic acid, poly-glutamic acid, and the like. Side chainfunctionalities can be used to build functional group-rich scaffolds foradded signal capacity or complexity.

FIG. 33A shows the structure of poly-lysine. In the tether constructsembodiments described in FIG. 33B through 33D, the poly-lysine tethersegments create a scaffolding for reporter attachment. In FIG. 33B, theamino groups of the lysine side chains (indicated by arrows) providefunctionality for attachment of pluralities of reporter elements to asubstrate construct, amplifying the reporter code. FIG. 33C illustratesa starburst dendrimer attached to a substrate construct) withpoly-lysine side chains (arrows).

FIG. 33D illustrates loading of the dendrimer oligomers that can bedetected by adding tagged complementary oligomers in a post-assemblylabeling and “amplification of signal” step. This provides a usefulmethod for preparing a universal tether—by attaching an untaggeddendrimer complex with multiple oligomeric reporter groups to a probe,and then treating the probe-bound dendrimer with a selection of one ortwo complementary tagged probes, a “painted” dendrimer specific for theindividual substrate species is obtained. Different probe/tetherconstructs can be painted with different complementary tagged probes.

In a further embodiment of this approach, the backbone of the reportersystem comprises eight unique oligonucleotides which are spatially codedin a binary fashion using two discernable fluorescent reporters, eachexcited via the same FRET donor. Prior to or following coupling of thetether constructs to its respective substrate construct, the tetherconstructs are sequence coded by hybridizing the appropriate mixture offluorescent reporter elements to create the proper probe-specific binarycode. Variations of this approach are employed using coded, unlabeleddendrimers, polymers, branched polymers, or beads as the backbone of thereporter system. Oligonucleotides can again be used for the binarycoding of the reporter construct. An advantage with this approach isthat signal strength is significantly amplified and that coding is notreliant upon a single hybridization event, both of which decrease thepossibility of measurement and/or coding error.

Still another embodiment replaces the above-described oligonucleotidecoding strategy with affinity-bound heterospecific ligands to produce,for example, a similarly binary coded reporter construct. Using a 9 bit,binary coding strategy, this non-universal reporter construct, in itssimplest form, employs only a single coupling chemistry tosimultaneously label all of the tethers.

Given the flexibility of the SBX approach, a broad range of reportersare used to produce unique, measurable signals. Each tether is uniquelyencoded by one or many distinct reporter segments. The scaffolding towhich the reporter moieties are attached can be constructed using abroad range of existing structural features including, but not limitedto dendrimers, beads, polymers, and nanoparticles. Depending on thecoding scheme, one or many distinctly separated reporter scaffolds canbe used for the reporter code of each tether. Any number of options areavailable for direct and indirect attachment of reporter moieties to thereporter scaffolding, including (but not limited to): reporter coding ofchemically reactive polymer(s) integrated into the tether constructs;reporter coding of chemically reactive surface groups on dendrimer(s)integrated into the tether backbone; and reporter coding of chemicallyreactive surface groups on bead(s) integrated into the tether. In thiscontext, a “bead” is taken broadly to indicate any crystalline,polymeric, latex, or composite particle or microsphere. For all threeexamples, reporter abundance can be significantly increased byattaching, to the reporter scaffolds, polymers that are loaded withmultiples of reporters. These polymers can be as simple as a 100 residuepoly-lysine or more advanced, such as labeled oligomeric probes.

Size reduced tether constructs can also be used. For example, the tetherconstructs can be lengthened in a post-processing step using directedmethods for inserting spacing units, thus reducing the size of thereporter tether.

Reducing the size and mass of the substrate construct can also beachieved by using unlabeled tethers. By eliminating bulky reporters (andreporter scaffolding such as dendrimers, which for some encodingembodiments comprise over 90% of the tether mass), hybridization and/orcoupling kinetics can be enhanced. Post-assembly tether labeling canthen be employed. Reporters are bound to one or more linkage chemistriesthat are distributed along the tether constructs using spatial orcombinatorial strategies to encode the base sequence information. Asimple binary coding scheme can use only one reactive linkage chemistryfor Xpandomer post-assembly labeling. More complicated labeling schemes,which can require hundreds of unique linkers, use an oligonucleotidebased strategy for Xpandomer labeling. Another post labeling Xprobe orXmer embodiment is to use the resulting nucleotide sequences derivedfrom P¹ and P² (see FIG. 10) that remain following cleavage andexpansion of the Xpandomer for reporter attachment by hybridization of alibrary of labeled probes. Similarly, other labeling and/or detectiontechniques can identify the more spatially resolved nucleotidesdirectly.

The tethers and reporter constructs are employed that attach to thesubstrate constructs with a desired level of accuracy since inaccuratecoupling leads to inefficiencies in detection, and can also lead topolymer termination or scrambling of the reporter code (e.g., if anasymmetrical reporter code is used). The fidelity of the SBX process canbe linked to the synthesis purity of the substrate constructs. Followingpurification of the tether/reporter construct to enrich for full lengthproduct, the construct can be directly coupled to a heterobifunctional(directional reporter coding) or homobifunctional (symmetrical ororiented reporter coding) oligonucleotide probe. As with all methods ofpolymer synthesis, purification (size, affinity, HPLC, electrophoresis,etc.) is utilized following completion of substrate construct synthesisand assembly to ensure high purity of full length, expandable probeconstructs.

Synthesis of Class I Substrates Constructs Displaying Reporters

Synthesis of Class I substrate constructs with reporters or reporterprecursors displayed on the tethers can be accomplished in a variety ofways. A stepwise process can be used to assemble a hairpin tetherpolymer that is connected near the tether attachment ends of thesubstrate construct via a disulfide bridge. This orients the reactiveends such that coupling of the tether to the probe is highly favored.Amino-Modifier C6 phosphoramidites are commercially available for allfour nucleotides (Glen Research, USA) and are used to attach the tetherto form the completed substrate construct, for example. Alternatively,linker chemistry in the form of benzaldehyde modified nucleotides can beemployed. Size and/or affinity purification is useful to enrich forcorrectly assembled substrate constructs.

Probe heterobifunctionalization can advantageously be done on a solidsupport matrix, as is customary for oligo and peptide synthesis, or insolution with appropriate purification methods. A broad range ofoff-the-shelf heterobifunctional and homobifunctional crosslinkingreagents are available for modifying, for example amine, carboxyl,thiol, and hydroxyl moieties, and to produce a variety of robust andselective linking chemistries. Since C6 amino modifiers are availablefor all four deoxy ribonucleotides, and can be made available for allfour ribonucleotides, functionalization strategies described here useoff-the-shelf amine based crosslinking methods along with wellestablished amine protection/deprotection chemistries. However, giventhe broad range of phosphoramidite and crosslinking chemistries known inthe art, methods not described here can be also considered and result inequivalent products.

The need for heterobifunctionalization of the probe can be eliminated ifthe reporter coding strategy produces digitally symmetrical coding oruses directional landmarks (parity bits) to identify code orientation.In this case, two internal amine probe modifiers are sufficient sinceeither coupling orientations of the reporter constructs on the tetherwould produce a unique, probe specific sequence identification.

One or many polymer segments can be sequentially assembled usingchemical (e.g., crosslinkers) or enzyme (e.g., nucleic acid probehybridization and ligation) catalyzed covalent linkages to form acircular, end functionalized tether. Given the current state of the artfor polymer synthesis methods, the chemical crosslinking synthesisapproach constitutes a representative embodiment. As is customary formany polymer synthesis methods, a solid support matrix can be used as ascaffold for synthesis. Polymer segments can be assembled one at a timeon the basis of end functionalization, as mixed pair segments havingdifferent end functionalizations, or as bound pairs two polymer segmentswith different homobifuntional end moieties (e.g., hydrazide and amine)paired via disulfide bridges.

Labeling chemistries, which includes both the linker and reporterelement moieties, are developed and optimized on the basis of highsignal yield and stability, low polymer cross reactivity andentanglement, and the structural rigidity (stiffening) which thesechemistries impart to the Xpandomer backbone, which can be important forsample preparation and detection as discussed below.

In FIG. 31 discussed above, the complete hairpin tether/reporterconstruct is assembled independently of the oligonucleotide probe andthen joined by homobifunctional or heterobifunctional linker chemistriesto the probe member. In an alternative embodiment, as shown in FIG. 34,tethers are circularized stepwise by construction on immobilized probesequences. The reporter construct and tether are synthesized withheterobifunctional (directional) or homobifunctional (symmetrical ororiented) tether segments linked directly to an oligonucleotide probe.The probe sequence shown in FIG. 34A is a 4mer and includes probemoieties P¹ and P² (the second and third circles) separated by aselectively cleavable bond (“V”). Solid state synthesis techniques areused in synthesis of the reporter construct. This synthesis isintegrated with closure of the tether loop. In Step I of FIG. 34A, afirst tether segment (341) with first reporter group (342) is addedusing specific functional group chemistry denoted by L1 and L1′ (linkerL1 on one of the probe moieties is blocked, as depicted by the smallrectangle). In Step II, a second tether segment (344) with secondreporter group (345) is added using specific functional group chemistrydenoted by L2′ and L2′. In Step III, a third tether segment (346) withreporter group (347) is added using specific functional group chemistrydenoted by L2 and L1′. In Step IV, and following removal of the blockinggroup from the L1 site on the P² moiety of the probe (again depicted bythe small rectangle), the loop is closed after L1′ and L1 coupling.

In yet another embodiment, as illustrated in FIG. 34B,heterobifunctional linker chemistry can again be used to ensure thetether is directionally positioned on the probe (although this is notnecessary for all encoding strategies). A probe 4mer is again depictedwith probe moieties P¹ and P² (the second and third circles) separatedby a selectively cleavable bond (“V”). In Step I, two tethers (341,344)with reporter segments (342,345) are contacted with functional groups L1and L2 on P¹ and P²; the chemistries are specific for each tether. Thetethers at this stage may be stabilized with intra-tether bonds(depicted by the adjoining triangles). In Step II, a third tether (346)with reporter segment (347) is used to “cap” the tethers after removalof blocking groups (shown as the small rectangles) on the predecessorsegments. The cap segment can also be stabilized with an intra-segmentbond (again depicted as adjoining triangles).

Turning now to FIG. 34C, an embodiment based on addition of probemoieties P¹ and P² to separate end linkages of a preformed tether isshown. In step I, the preformed tether is first reacted with P¹ bycontacting L1 to L1′. In Step II, the tether is then reacted with P² bycontacting L2 to L2′. The tether may be stabilized by an intra-tetherbond (depicted by adjoining triangles), which brings P¹ into proximitywith P². The two probe moieties are then ligated in Step III to form theselectively cleavable bond (“V”) between P¹ and P² (the second and thirdcircles). P¹ and P² ligation may optionally be facilitated by duplexingsaid probe moieties to a complementary template.

In FIG. 35A, another embodiment is discloses for synthesis of a reporterconstruct segment. Using solid state chemical methods, a cleavablelinker (351) is first anchored on a solid substrate (350). A firsttether segment with a reversible linkage (352) is reacted with thelinker in Step I, and then in Step II reacted with a combinatoriallibrary of monomers M1 and M2, shown in this example as a 4:1stoichiometric mix of the respective monomers. Random copolymersynthesis is done in this manner to produce unique tether segmentcompositions. If peptide or amino acid monomers are used, this can bedone with mixed anhydride chemistry, for example, resulting in randomcopolymer peptide tethers of variable length (Semkin et al., “Synthesisof peptides on a resin by the mixed anhydride method”, Chemistry ofNatural Compounds 3(3):182-183,1968; Merrifield et al., “Solid PhasePeptide Synthesis. I. The Synthesis of a Tetrapeptide”. J. Am. Chem.Soc. 85(14):2149-2154,1963). In Step III, an end linker element (356)blocked at L1 (depicted as a small rectangle) is then added to thepeptide segment. Following cleavage of the segment from the solidsupport (not shown), the tether segment can be incorporated into areporter construct using a variety methods, some of which were describedin FIG. 34.

In FIG. 35B, an alternative to this embodiment is demonstrated. Areporter construct with randomly incorporated peptide fragments issynthesized as before in Steps I, II and III, but in Step III the endlinker group (358) is provided with a heterofunctional linker L2.Different from the previous example, monomers are provided in equalproportions. After cleavage from the solid support (not shown), thereporter construct is available for further incorporation into tethersof the previous examples and is a heterobifunctional linker. Reporterconstructs serve to encode the genetic information of the probe sequencefragment, as is described in more detail below.

Polypeptides are useful reporter constructs and also serve as tethers.Random, periodic, alternating, and block copolymers along withhomopolymer, for example, can be utilized for tether segment and tetherconstruct composition. Polypeptide segments can be end functionalizedusing succinimidyl containing heterobifunctional crosslinkers for amineconversion to a hydrazide or 4-formylbenzoate (4FB), for example.Polypeptides can be produced either by chemical synthesis or by cloningand over expression in biological systems (bacteria, yeast,baculovirus/insect, mammalian). Depending on the desired length of thesegment, tether segments can range from N>2 (short segments) to N>1000(long segments). Amine side group protection chemistries may be used asappropriate. As an alternative to using off the shelf crosslinkers,polypeptides can be chemically synthesized with hydrazide, 4FB, and NHSmoieties directly attached.

Polypeptide segments end functionalized using maleimido-containingheterobifunctional crosslinkers for thiol conversion to a hydrazide or4-formylbenzoate (4FB), can also be used in synthesis of tethers andreporter constructs. Polypeptide segments end functionalized using EDCcrosslinkers for carboxyl conversion to a hydrazide (HZ) or4-formylbenzoate can similarly be employed.

Synthesis of all five classes of oligomeric substrate construct havingtethers modified with reporter constructs is achieved by use of theabove synthetic methods. Similarly, synthesis of monomeric substrateshaving tethers modified with reporter constructs may also be achieved bythe above synthetic mechanisms. The chemistries for these variants aregenerally applicable to the genera of Xpandomer species shown in FIGS. 8and 9. We now turn to encoding strategies and rules for conveyinggenetic information in Xpandomers with reporter constructs.

Reporter Constructs and Reporter Code Strategies

A “reporter code” is a digital representation of a particular signal orsignal sequence that is embodied in the reporters of a particularreporter construct. Whereas the “reporter construct” is a physicalmanifestation of extragenetic information, the reporter code is itsdigital equivalent.

Digital encoding requires reporter codes to follow certain rules. Forexample, at least 256 reporter codes are required to identify all thepossible combinations of a 4mer Xprobe library. Having more reportercodes than there are possible reporter construct combinations isadvantageous because extra states can be used for other purposes such astagging of gaps, providing positional information or identifying parityerrors or high order errors.

A number of strategies can be considered for physically representing areporter code. The tether can be divided into one or many codablesegments, each of which can be labeled either before or after Xpandomerassembly. Variable signal levels (amount of label), lengths (duration ofsignal), and shapes of labeled tether constructs segments can be used toincrease coding options. Coding can also be expanded by usingmultiplexable labels. For instance, using a mass tag label approach, abroad library of spectrally distinct tags can be used to uniquely encodea single reporter segment; 14 distinct mass tags used in combinations ofthree tags clustered on a single tether constructs segment would create364 unique 3-mass spectra. For multi-segmented tether, post Xpandomerassembly labeling of most or all of the tether backbone can have theadded benefit of increasing the rigidity of the Xpandomer, potentiallymaking it easier to manipulate for detection and improving stability.

FIG. 36A (and also FIG. 2A) illustrates a tether with a single reportersegment of an Xprobe or an Xmer. This approach benefits from highlymultiplexable reporter labeling, such as mass spectrometer tags, toproduce a broad library of spectrally distinct outputs. A cleavable masstag is a molecule or molecular complex of cleavable reporters that canbe readily ionized to a minimum number of ionization states to produceprecise mass spectra. When carefully controlled, a mass spectrometer candetect as little as a few hundred such mass tag reporters. This reportercode example needs no positional information of the reporter on thetether backbone to determine the code state (though positionalinformation is required to distinguish one reporter code from the next).This characteristic simplifies the tether construction and potentiallyshortens the tether length requirements.

In one embodiment, using cleavable mass tag labels, the lengthenedXpandomer can be presented for detection via a nanopore ion source(electrospray ionization, atmospheric pressure chemical ionization,photoionization) or by surface deposition (nanocomb, nanochannel,laminar flow, electrophoretic, and the like) followed by desorptionionization with laser, ion beam, or electron beam sources (MatrixAssisted Laser Desorption Ionization “MALDI”, Desorption ElectrosprayIonization “DESI”, Desorption Ionization on Silica “DIOS”, Secondary IonMass Spectrometry “SIMS”).

An example of a 9-bit tether constructs with 2 detection states (“1” and“0”), which produces 512 code identities, is described in FIG. 36B. Inone embodiment, the tether constructs consists of segments “1” and “0”that produce two levels of electrical impedance as measured in aCoulter-like nanopore detector. In a second embodiment, the tetherconstructs consists of electrical conducting segments “1” andnon-conducting segments “0”. In a third embodiment the tether constructsconsists of fluorescent segments “1” and non-fluorescent segments “0”. Aplurality of different reporter elements can be considered for this typeof coding. For any of these approaches, the tether assembly and probeattachment chemistry can be identical—only the reporter segmentcomposition would need to change. With this simple format, labeling canbe done either before or after Xpandomer assembly. Post-labeling isdesirable since the unlabeled reporter segment is significantly lessmassive and, as such, is usable at a much higher concentration than afully labeled reporter construct. Depending on the strategy, the polymersegments can be: (1) coded via conjugatable or reactive surfacechemistries (e.g., poly-lysine, poly-glutamic acid), (2) non-reactive(e.g., PEG, low reactivity polymers), or (3) a mixture of both reactiveand non-reactive polymers. Reactive groups include, but are not limitedto, primary amines (—NH₃), carboxyls (—COOH), thiols (—SH), hydroxyls(—OH), aldehydes (—RCOH), and hydrazide (—R—N—N) moieties. Labeling ofreactive segments, which may include reactive group deprotection, can bedone directly on the substrate constructs, after formation of theXpandomer intermediate, after backbone cleavage to produce Xpandomer, orat any other time in the SBX process as appropriate to produce the bestresults.

Additional levels may be possible, as shown in FIG. 36C. For example, abinary, directional (non-symmetrical reporter construct) coding strategyrequires at least eight reporter coding segments and a ninth segment, acodable capping segment (for tether loop closure and as possible centersubstrate construct landmark) to produce the minimally required 256codes for a four base substrate (512 codes if the capping segment iscoded).

Turning to FIG. 36D, a 7-bit reporter construct, each reporter withthree detection states, is shown to result in 2187 detectable codeidentities. Use of flexible polymer spacers may be used for stericreasons.

In FIG. 36E, a rigid equivalent of the previous 7-bit reporter constructis shown.

FIG. 36F describes an example of a rigid, 4-segment tether constructswith eight total distinct states or label combinations per segment.Using seven of the label combinations to label three of the tetherconstructs segments will produce 343 unique code identities and willleave 1 segment available for subunit boundary identification, parity,or other functional purpose. This embodiment can use a mixed labelingapproach where 1 to 3 different labels can be incorporated on eachsegment to produce at least eight unique combinations per segment asshown in FIG. 37, where combinations of different reporter types andreporter construct chemistry are contemplated. Mass tag and fluorescentlabeling options, among others, can be utilized as described. PostXpandomer assembly tether constructs labeling is directed by theabundance and identity of three crosslinker moieties as described inFIG. 37. Tether segment lengths can be on the order of 100-1000 nm fordiffraction limited measurements or <100 nm for near field measurements,if it is desired to use these detection technologies. Shorter tetherscan be used for other detection methods.

A rigid, 3-segment tether constructs with 22 total distinct labelcombinations per segment is described in FIG. 36G. Using 21 of the labelcombinations described in FIG. 37 to label two of the tether constructssegments will produce 441 unique tether constructs identities and willleave one segment available for Xprobe subunit boundary identification.This embodiment uses a mixed labeling approach where one to threedifferent labels can be incorporated on each segment to produce up to 22unique combinations per segment (FIG. 37). Mass tag and fluorescentlabeling options, among others, can be utilized for this embodiment. Asdescribed in FIG. 37, post Xpandomer assembly tether construct labelingis directed by the abundance and identity of three chemical moieties.

By designing reporter/tether segments in which the reactive groupabundance and spatial dimension (radial distance from polymer backbone)can be varied, coding levels can be achieved wherein at least threetotal code states are possible: High “2”, Medium “1”, and Low “0” (FIG.36H). On the other hand, a three label, two level, coding strategy (i.e.21 states per reporter) would require only two reporter coding segmentsto produce 441 codes and could use an additional segment for codeorientation (FIG. 36I).

Reporter coding can be designed to reduce errors inherent with thereporter construct and associated detection technologies using severalapproaches. In the case of nanopore Coulter-like detection, the velocitythat an Xpandomer passes through the pore and the current modulation itcauses can depend on many factors including, but not limited to, thecharge state of the portion of the Xpandomer within the pore, theelectrolyte concentrations, the nanopore surface charge states, theapplied potential, frictional effects limiting Xpandomer movement, andthe relative dimensions of both the Xpandomer and the nanopore. If thevelocity is not predictable, decoding the current modulation cannot usetime (and constant velocity) to resolve the reporter measurementassignments. One encoding embodiment solves this issue using 3-stateencoding. The reporter's signal is the impedance that a label causes tothe electrolyte conductivity through the nanopore. By providing threepossible levels of impedance for a reporter, one bit of information isencoded by the transition to the next label. By design, this transitionis always a change to one of the other two states. If the three statesare labeled A, B and C, then a sequence of reporters never has 2 As, 2Bs, or 2 Cs together. In this way, information is encoded in leveltransitions and is therefore independent of velocity through thenanopore. One encoding scheme is to assign all transitions A to B, B toC, and C to A a value of “0” whereas transitions B to A, C to B and A toC are assigned the value “1”. For example, the detected sequence ABACBCAdecodes to 0, 1, 1, 1, 0, 0.

Although timing cannot reliably resolve sequential labels, it can besufficient to differentiate the separation of the label sequence on asingle tether constructs from that of the next sequential tether.Additional spacer tether lengths at either end of the reporter labelsequence (at the substrate construct attachment points) can provide alarge timing gap that delineates the tether constructs codes in time.

In cases where such timing is insufficient, a frame shift error canoccur. Frame shift errors result when the detector reads a series oflabels from multiple tether constructs codes (frames) but does notcorrectly delineate the start-of-code (start of frame). This results inwrong codes. One embodiment to solve this is to add more bits in thecode than are needed to identify the corresponding base sequences (whichare typically 1 to 4 bases long). For example, eight bases are requiredto uniquely identify a 4 base sequence. Each 2 bit pair describes asingle base. By adding a parity bit for each base, the tether constructscode is increased to 12 bits. A high parity error rate (near 50%) wouldindicate a frame shift error that would result from a missed statechange. Besides a missed state change, another error type that can occurin this nanopore detection technology is that of a misread state. Singlereporter errors can be isolated to the particular base using the paritybit and it can then be assigned the value “unknown base”.

The Xpandomers labeled using either electrically impeding, electricallyconducting or fluorescent segments can be measured in solution using avariety of nanopore, nanocomb, or nanochannel formats. Alternatively,the Xpandomers can be surface deposited as spatially distinct, linearpolymers using nanocomb, nanochannel, laminar flow or electrophoreticdeposition methods, among others. As with the solution approach, directdetection of surface elongated Xpandomer can be done by measuring thesignal characteristics of labeled tether segments. Depending on thecomposition of the deposition material (conducting, insulating) and theunderlying substrate (conduction, insulating, fluorescent), a variety ofdetection technologies can be considered for label detection.

Further SBX Methods

A number of SBX methods using class I-V oligomeric substrate constructswere illustrated in the previous figures (FIGS. 11-17, 20, 21, 24). Wenow consider optional methods that supplement these protocols.

End Functionalization

FIG. 38 illustrates preparation and use of target end adapters. FIG. 38Aillustrates end functionalized complementary oligonucleotides duplexedto form an adapter with a bifunctional, conjugatable end (“L1” and “L2)and an enzymatically ligatable end (5′ phosphate and 3′-OH). Theseadapters can also be designed with additional functionalities. Forexample, as shown in FIG. 38B, end functionalized adapters with nested,backbone crosslinkers (“L3”) and a cleavable bond (“V”) can besynthesized. For simplicity, the cleavable bond V can be the samecleavable bond chemistry (or enzymology) used to release or expand theXpandomer, although other cleavable linkers can be utilized if desiredto differentiate between independent cleavage steps. FIGS. 38C and 38Dshow steps for construction of a multifunctional adaptor of FIG. 38B. Asshown, a magnetic bead with a surface tethered oligonucleotidecomplementary to each adapter strand (two different bead mixes) can beused to assemble differentially modified oligonucleotide segments. Onceassembled, segments can be enzymatically ligated to covalently link theannealed segments. With this approach, each segment can be modifiedindividually in a manner not available to standard oligonucleotidesynthesis.

Manipulation of the Xpandomer can be useful for efficient samplepresentation and detection. For example, terminal affinity labels can beused to selectively modify one end of the Xpandomer to allow forelectrophoretic elongation. Attachment of a bulky, charge neutral,modifier to either the 3′ or 5′ end (not both) produces anelectrophoretic drag on the Xpandomer that causes the non-modified endto elongate as it travels to the detector. End modifiers, which includebut are not limited to microbeads, nanoparticles, nanocrystals,polymers, branched polymers, proteins and dendrimers, can be used toinfluence the structure (elongation), position, and rate at which theXpandomer is presented to the detector by imparting unique,differentiating properties to its termini such as charge (+/−/neutral),buoyancy (+/−/neutral), hydrophobicity, and paramagnetism, to name afew. In the provided examples, the end modifications produce a dragforce that enables the Xpandomer to elongate; however, the oppositestrategy can also be employed wherein the end modification is used topull the Xpandomer towards and through the detector. With this approach,pulling of the end modifier facilitates Xpandomer elongation. The endadaptor on the template strand may optionally also contain one or morenucleic acids that will be used to synthesize frame registration andvalidation signals in the finished Xpandomer (see FIG. 54).

Incorporation of an affinity modifier can be done either prior to,during, or after Xpandomer synthesis (hybridization, ligation, wash,cleavage). For example, terminal affinity tagged primers that arecomplementary to adapter sequence can be pre-loaded to ssDNA targetunder highly specific conditions prior to Xpandomer synthesis. Theprimer and its affinity tag can be incorporated into the full lengthXpandomer and can be used to selectively modify its end. A potentiallymore elegant approach is to enzymatically incorporate end modifiers.Terminal transferase (TdT), for example, is a template independentpolymerase that catalyzes the addition of deoxynucleotides to the 3′hydroxyl terminus of single or double stranded DNA molecules. TdT hasbeen demonstrated to add modified nucleotides (Biotin) to the 3′terminus (Igloi et al., “Enzymatic addition of fluorescein- orbiotin-riboUTP to oligonucleotides results in primers suitable for DNAsequencing and PCR”, BioTechniques 15, 486-497, 1993). A wide range ofenzymes are suitable for this purpose, including (but not limited to)RNA ligases, DNA ligases, and DNA polymerases.

In FIGS. 38E and 38F, hairpin adaptors are illustrated. Hairpin adaptorsfind use in blunt ended ligation to yield self-priming template strands.One advantage of this approach is that the daughter strand remainscovalently coupled to the template strand and more quickly re-annealsfollowing a melt to remove unligated material and low molecular weightfragments. As shown in FIG. 38F, these adaptors can contain embeddedpre-formed linker functionalities for purification or downstreamhandling, and can also contain cleavage sites for more efficient harvestof the Xpandomer daughter strands.

Target Template Preparation and Parsing

To perform whole genome sequencing of long, continuous DNA using theXprobe based SBX methods presumes that the DNA is prepared in amanageable form for hybridization, ligation, gap filling if required,expansion and measurement. The Xpandomer assembly process can, in someembodiments, be improved by surface immobilization of the DNA target inorder to (1) reduce complexity and cross hybridization effects, (2)improve washing, (3) enable target manipulation (elongation) in order tofacilitate improved hybridization, and/or (4) if nanopore sensor isused, provide a seamless interface with the detection process. Asdescribed in detail below, methods for target template preparation,parsing, and surface attachment is expected to improve data quality andsequence assembly.

Most whole genome sequencing methods require fragmentation of the targetgenome into more manageable pieces. The largest chromosome in the humangenome (Chromosome 1) is ˜227 Mb and the smallest chromosome (Chromosome22) is ˜36 Mb. For most of the highly processive and continuousembodiments described herein, 36 Mb of continuous sequencing is far toolong to be sequenced with high efficiency. However, to take advantage ofthe inherent long read length capability of SBX, DNA fragment lengthsof >1 kb are targeted. Accordingly, a number of strategies can beemployed to accomplish genome fragmentation and prepare a DNA target setcompatible with the SBX method as disclosed herein.

One embodiment involves fragmenting the total genome into 1-10 Kb pieces(average 5 Kb). This can be done either by restriction enzymes or byhydrodynamic/mechanical shearing. The fragments are then blunt ended andrepaired in preparation for blunt end ligation to a Sequence Adapter(“SA”) or a Sequence Adapter-Dendrimer (“SAD”) construct. The non-bluntend of the SA or SAD construct is designed to be incapable of ligationso as to prevent multimer assembly of said adapters. Any SA or SADligated target that is present can be affinity purified away from freeadapters. At this point, if the capture dendrimer is not introduced withthe SA construct then this can be done (using an efficient excess ofdendrimer) followed by purification to isolate the target-SAD complex.Once purified, the complex is ready for attachment to the assay surface.To this end, it is desired that only a single target complex be attachedper reaction location. Alternatively, the capture dendrimer can insteadbe associated with the assay surface, in which case the purifiedtarget-SA complex can be attached directly to the dendrimer that isalready located and covalently attached to the surface. A similar methodfor DNA target assembly on surfaces is been disclosed by Hong et al.(“DNA microarrays on nanoscale-controlled surface”, Nucleic AcidsResearch, 33(12): e106, 2005).

Another approach, called the “parsing” method, involves coarselyfragmenting the genome into 0.5-5 Mb pieces (using rare cuttingrestriction enzymes or by hydrodynamic/mechanical shearing), followed bycapture of such fragments to a modified microarray (or partitionedsurface) composed of gene/loci specific oligonucleotide capture probes.Custom microarrays and oligonucleotide capture probe sets are broadlyavailable from a number of commercial sources (Arraylt, Euorfins-Operon,Affymetric Inc.). This additional parsing can provide an advantage forbackend sequence assembly. Capture of these large fragments involveseither partial or complete target denaturation in order to allow thecapture probes to bind or duplex with specific target DNAs. To reducenon-specific hybridization it can be necessary to load the capture arrayunder dilute conditions so as to prevent cross hybridization betweentemplates.

Each partitioned capture probe set, which can be arrayed on a largesurface, is designed to provide linear genome resolution of ˜3 Mb. Toprovide efficient genome parsing, each individual capture probe set canbe composed of 3 to 5 gene/loci specific oligonucleotides, linearlyseparated on the genome by 0.5-1.5 Mb each. The capture probes areselected to be completely unique to the target fragment, thus providingboth specificity and redundancy to the method. Given 3 Mb resolution ofa 3 Gb genome, this approach requires a capture array composed ofapproximately a 1000 gene/loci specific probes. Microarray readyoligonucleotide capture probe sets specific to human gene targets arebroadly available off the shelf (Operon Biotechnologies, HuntsvilleAla., USA).

Once non-specific binding events have been washed away, each capturearray can then proceed as independent reactions through the remainder ofthe genome preparation process in the same manner as discussed above forthe non-parsing method. The primary difference being that the targetsample can now be positionally parsed on an SBX assay surface or asindividual solution based reactions, thus reducing the complexity ofpost data acquisition sequence assembly.

Another embodiment is to have non-tethered Xprobe-target hybridizationand Xpandomer production. In this case, the SBX assay is performed infree solution. This approach can use one or a combination of physicalmanipulations such as using electrophoretic, magnetic, drag tags, orpositive/negative buoyancy end functionalities under static or laminarflow conditions, as a means to elongate the target DNA prior to andduring probe hybridization and ligation, for example, and, to lengthenor expand the cleaved Xpandomer prior to detection. Free solutionsynthesis of Xpandomers, without immobilization, can be done usingpolymerases and ligases (with and without primers) and can also be doneusing chemical ligation methods. Both triphosphate substrate constructsand monophosphate substrate constructs can be used. Simultaneoussynthesis of Xpandomers from multiple and mixed nucleic acid targets isconceived. Generally, substrate triphosphate constructs are capable ofcontinuous, processive polymerization in solution and can be adapted tosingle-tube protocols for massively parallel single-molecule sequencingin free solution, for example.

Surface Assembly of Nucleic Acid Targets

One method of preparing target nucleic acids for sequencing uses endfunctionalized double stranded DNA target as shown in FIG. 39. For thisexample, each adaptor is typically provided with an ANH group (reactivehydrazide) useful for further processing, for example, usingsuccinimidyl 4-hydrazinonicotinate acetone hydrazone (SANH) linkagechemistry and an amine modified end adapter oligonucleotide duplex.Similarly, an amine reactive SANH can be used to create reactivehydrazide moieties. SANH readily conjugates with aldehydes like4-formylbenzoate (4-FB) to form stable, covalent hydrazone bonds. Aminereactive C6-succinimidyl 4-formylbenzoate (C6 SFB) can be used to createa reactive benzaldehyde moiety. End functionalized complementaryoligonucleotides are duplexed to form a bifunctional, conjugatable end(SANH and amine) and an enzymatically ligatable end (3′ OH and 5′phosphate). Ligation of adapter creates an end functionalized dsDNAtarget that can be crosslinked to surface tethered aldehyde groups. FIG.39 illustrates dsDNA target end functionalized using SANH and aminemodified end adapter oligonucleotide duplex.

For many of the described SBX methods, nucleic acid targets may becovalently tethered to a flat, coated solid support (stainless steel,silicon, glass, gold, polymer). As described in FIG. 40, targetattachment points can be produced by 4FB derivatization of a SAMmonolayer. The method utilizes the ANH adaptor of FIG. 39, and in Step Iof FIG. 40, the ANH is reacted with the 4FB heads of the monolayer andthe template is denatured (Step II). The other strand of the template,which is also end labeled, can be captured in a separate reaction. InStep III, the free amine on the 3′ end of the single-stranded templateis then reacted with a bead, for example, here shown as a buoyancy bead,so that the target template can be stretched. These capture complexescan be assembled either by random self assembly of a stoichiometricallybalanced mix of end functionalized polymers (ex. thiol-PEG-hydrazide fortarget attachment; thiol-PEG-methoxy for Self Assembled Monolayer “SAM”capping) or by patterning spatially resolved reactive attachment pointsusing lithographic techniques. The patterned lithographic method canproduce consistently spaced target attachment points, although this isdifficult to do for single molecule attachment, while the random selfassembly method would likely produce more variable target attachmentspacing, but has a high percentage of single molecule attachments. Abroad range of monofunctional, bifunctional, and heterobifunctionalcrosslinkers are commercially available from a variety of sources.Crosslinker compatible monofunctional, bifunctional, andheterobifunctional polymers (polyethyleneglycol, poly-l-lysine) are alsoavailable from a wide range of commercial sources.

DNA target density of 1 Billion targets on a 100 cm² surface wouldrequire an average per target area of 10 um². Target spacing in thisrange provides sufficient target separation to prevent significant crossreaction of bead tethered 5000 base long target nucleic acids (100-1000nm bead diameter; 5 Kb dsDNA=1700 um). Target area can be easily beexpanded if target and/or bead (>1 bead/target) cross reactivity isdetermine to be unacceptably high.

Target Elongation Using Beads or Nanoparticles

As shown in FIG. 40, a bead or nanoparticle (400) tethered on the freeend of target DNA can be utilized to elongate and hold target in itssingle stranded conformation during, for example, Xprobe libraryhybridization. Retaining the target in an elongated conformationsignificantly reduces the frequency and stability of targetintramolecular secondary structures that form at the lower temperatures.The reduction or even elimination of secondary structure influencespromotes efficient, high-fidelity substrate construct assembly.

A variety of approaches for applying an elongational force to the singlestranded target may be employed. For example, paramagneticbeads/nanoparticles/polymers enable the use of a magnetic field todeliver controlled, directional force to the surface tethered target.Further, by controlling the direction of the magnetic field lines,paramagnetic beads/particles can be used to guide and hold the Xprobeloaded target on the substrate surface during wash steps. Bysequestering the targets along the surface, target loss due to shearforces can be minimized. As an alternative to magnetic field elongation,positive and negative buoyancy beads/particles that are either more orless dense than water, respectively, can be used to provide anelongational force. All beads/particles are surface coated as necessaryto minimize non-specific interactions (bead aggregation, probe binding)and functionalized to enable covalent crosslinking to adapter modifiedtargets.

FIG. 41A through 41D illustrates representative bead based targetelongation strategies. Target elongation using end tethered paramagneticbeads/particles attracted to an external magnetic field (B) isillustrated in FIG. 41A. Target sequestering on the substrate surface(to reduce target shearing) using end tethered paramagneticbeads/particles attracted to an external magnetic field is illustrate inFIG. 41B. FIGS. 41C and 41D illustrate target elongation using endtethered negative buoyancy beads/particles (higher density than water)and positive buoyancy beads/particles (lower density than water),respectively. Free solution methods for target elongation using endtethered moieties, for example using both a positive and negativebuoyancy bead to functionalize opposite ends of a target, provide anelegant alternative for reducing target secondary structure.

Use of these elongation strategies in preparation of Xpandomers isillustrated in FIG. 41E. Here, in Step I, the immobilized template (416)is stretched using a buoyant bead duplexed to the target via the adapterprimer (417). The template is then contacted in Step II with substrateconstructs and these are then ligated to the single stranded target,producing a double-stranded Xpandomer intermediate (411) withcharacteristic probe-loop construction and two backbones, one throughthe primary backbone of the polynucleotide and the other throughconstrained Xpandomer backbone. In Step III the template strand isdenatured, and in Step IV the single-stranded Xpandomer intermediate iscleaved at selectively cleavable bonds in the primary backbone,resulting in an unfolding of the loops and elongation of the Xpandomerproduct with fully extended surrogate backbone (419).

DNA Target Elongation Using Polymer End Modifications

An alternative to the methods described in FIG. 40 utilizes long,functionalized polymers (instead of beads) covalently linked to the freeends of surface tethered DNA targets to elongate target.Electro-stretching and threading of polymer end modifications throughporous substrate following by capture (sequestering) of polymer withinsubstrate produces fully elongated single stranded target DNAsignificantly free of secondary structure. FIG. 42A illustrates thismethod, which shows threading of the target strands through pores in asubstrate. Porous substrates include, but are not limited to, gelmatrix, porous aluminum oxide, and porous membranes. Capture of fullyelongated polymer can be achieved by controlled chemicalfunctionalization wherein crosslinking or binding of polymer to poroussubstrate can be selectively initiated after full elongation of target.A variety of crosslinking or binding strategies are available. Forexample, crosslinking of carboxyl functionalized elongated polymer toamine functionalized porous substrate can be achieved with theintroduction of the crosslinking agent 1-Ethyl-3-[3-dimethylaminopropyl]carbodiimide Hydrochloride (EDC).

A further embodiment is illustrated in FIG. 42B, is to electrostretchthe polymer towards a functionalized surface that crosslinks or binds tothe polymer. In this case, the polymer is not necessarily threadedthrough the substrate but instead crosslinks or binds to functionalgroups on the surface. For this approach, the substrates include, butare not limited to, gel matrix, porous aluminum oxide, porous membranes,and non-porous, conductive surfaces like gold. Both the porous andnon-porous substrates can be functionalized using a variety of off theshelf crosslinking (hydrazide-aldehyde; gold-thiol) or bindingchemistries (biotin-streptavidin),

As an alternative, an enclosed electrical gating system may be employedthat uses varying (high, low, ground) electric field regions to capturecharged polymer segments. The method, which is illustrated in FIG. 42C,produces a Faraday Cage type of enclosure which can be used to hold thepolymer and target DNA in its elongated position. This approach requireseffective partitioning or confinement of electric fields, and has theadvantage of allowing adjustable target elongation (throughout the SBXsynthesis) by modifying the gating parameters. Outside of the FaradayCage, electric field influences on substrate constructs are minimal.

In a related method, an elastic polymer is used to deliver a constantstretching force to the less elastic target nucleic acid. This methodcan be used as a supplement to any of the elongation methods describedhere, including magnetic bead, buoyancy deformation, density deformation(gravity), or electrostretching. The elastic force stored within thepolymer provides a more consistent, buffered elongational force to thetarget strand.

Gel Matrix Based Target Substrate with Electro Straightening

In FIG. 43A, shows an alternative method for single stranded target DNAarraying (versus attachment to solid support) in which the target iscovalently attached to a sieving gel matrix (430). In FIG. 43 b (insert)substrate constructs can be seen associating with the anchoredtemplates, which have been stretched straight. Electric fields can beused for target elongation and substrate construct presentation. Thisapproach has advantages over other methods for DNA target array in termsof target density and target elongation (electro-straightening). Forexample, DNA crosslinking to acrylamide via acrylate modified endadapters (attached to oligonucleotides) is routinely done. Acrydite™ isan available phosphoramidite (Matrix Technologies, Inc., Hudson, N.H.,USA) that has been extensively used to incorporate methacryl 5′ terminalmodifications to oligonucleotides: double bond in the Acrydite groupreacts with activated double bonds of acrylamide (Kenney et al.,“Mutation typing using electrophoresis and gel-immobilized Acryditeprobes”, Biotechniques 25(3):516-21, 1998). Amine functionalized agaroseis also available off the shelf (G Biosciences, St. Louis, Mo., USA) andcan similarly be used to crosslink to DNA using off the shelf aminereactive crosslinking chemistries (Spagna et al., “Stabilization of aβ-glucosidase from Aspergillus niger by binding to an amine agarosegel”, J. of Mol. Catalysis B: Enzymatic 11 (2,3): 63-69, 2000).

As with the previously described solid support attachment methods, DNAfragments are usefully functionalized with end adapters, to produce, forexample, a 5′-Acrydite modification. To ensure target uniformity, dsDNAtargets are denatured (and maintained in a denatured state) whilecrosslinking to sieving matrix. Target can be denatured using a varietyof existing techniques including, but not limited to, urea, alkaline pH,and thermal melt. Once crosslinked to a gel matrix, denatured target DNAcan be straightened by applying an appropriate electric field (similarto electrophoresis). Diffusion, electrophoresis, and neutralization ofthe denaturants within the gel matrix along with tight temperaturecontrol, with the addition of hybridization compatible solutions,produces an environment conducive to substrate construct hybridization.For example, Xprobes can be presented continuously to tethered targetsby electrophoresis. This format allows for probe recycling and providesgood control over Xprobe flow rate.

The target density of a 5 mm×50 mm×20 mm gel matrix array produces a5.0×10¹² μm³ reactive volume, which is substantially higher than whatcan be arrayed on a flat surface. FIG. 43C shows the chemical structuresfor acylamide, bis-acrylamide, and the Acrydite oligonucleotide endmodification. FIG. 43D illustrates a polyacrylamide gel matrix withcovalently attached DNA target.

Enzymatic processing of substrate constructs can be done within the gelmatrix. Enzymatic assays using polymerases (PCR) and ligases areroutinely used for modification of oligonucleotides covalently coupledto acrylamide Gel Pads (Proudnikov et al., “Immobilization of DNA inPolyacrylamide Gel for the Manufacture of DNA and DNA-OligonucleotideMicrochips”, Analytical Biochemistry 259(1): 34-41, 1998). Ligase, forexample, is delivered to assembled Xprobe-targets within the gel matrixeither by passive diffusion, electrophoresis, or a combination of both.A broad range of matrix densities and pore sizes are considered,however, passive diffusion benefits from relatively low density matrixand large pores. Active delivery of ligase by electrophoresis can beachieved by attachment of charge modifiers (polymers and dendrimers) tothe enzyme to enhance its migration through the matrix.

A variation of the gel matrix method is to electrophorese end-modifiedsingle-stranded target polynucleotides through a gel matrix. Endmodifications can be used to create drag on one end of the target. Acationic end modification can be performed on the other end of thetarget so as to pull it through the gel. A temperature gradient or zonecan also be induced in the gel so as to ramp or modulate hybridizationstringency as the targets progress through the gel matrix. Enzymaticprocessing can be done within the gel matrix or after the hybridizedtarget-substrate complexes exit the gel matrix. The process can berepeated as necessary to produce the desired average Xpandomer length.

FIG. 44 illustrates the use of drag tags for Xpandomer manipulationpost-synthesis. The drag tag (depicted as a diamond) serves as asolution-equivalent of a stretching and conveying technique (Meagher etal., “Free-solution electrophoresis of DNA modified with drag-tags atboth ends”, Electrophoresis 27(9):1702-12, 2006). In FIG. 44A, drag tagsare attached by linker chemistry to adaptors functionalized on the 5′ends of the single stranded template (L1′ of the drag tag combines withL1 of the template). In the step shown, processive addition of substrateconstructs is shown already in progress. In FIG. 44B, the drag tag isadded by annealing to a complementary adaptor. In FIG. 44C, theXpandomer is first treated so as to attach a linker to a 3′-adaptor(shown as the small square with L1′), and the linker (L1) is then usedin Step II to attach the drag tag thereto. Terminal transferase, whichextends 3′-OH terminus of single or double stranded DNA, polymerizeslinker-modified nucleotide triphosphates such as biotinylatednucleotides. Other enzymes can also be used to add modified bases oroligomers. In FIG. 44C, a drag-tag is added to the 3′ end of theXpandomer intermediate. Drag tags can include, but are not limited to,nanoparticles, beads, polymers, branched polymers and dendrimers.

Gaps and Gap Filling with Substrate Construct Hybridization and Ligation

Multiple variants on ligation methods can be employed to address gapmanagement and gap filling. In one embodiment, cyclical stepwiseligation is used, where probes are sequentially assembled from a surfacetethered primer duplexed to the target polynucleotide. In anotherembodiment, “promiscuous” hybridization and ligation of substrateconstructs occur simultaneously across the entire target DNA sequence,generally without a primer.

For both the cyclical and promiscuous ligation approaches, it ispossible that, for example, some portion of 4mer substrate constructs(e.g., 25%) and some portion of 5mer substrate constructs (e.g., 20%)will hybridize adjacently so as to allow for ligation. Of these adjacentduplexes, some percentage may contain mismatched sequences that willresult in either measurement error (if ligated) or failure to ligate.Most instances of ligation failure (due to mismatch) is withoutconsequence since unligated probes are removed prior to the next roundof hybridization. The hybridization/wash/ligation/wash cycle can berepeated several hundred times if desired (6 minutes/cycle=220cycles/day and 720 cycles/3 days; 10 minutes/cycle=144 cycles/day and432 cycles/3 days).

In the cyclical process (illustrated with Xprobes in FIG. 14), Xprobesare sequentially assembled from one end of a surface tethered primerduplexed to the target DNA; one Xprobe per cycle. This being the case,the target read length can be calculated using the followingassumptions: 25% adjacent hybridization (of 4 mers) with 20% perfectprobe duplex fidelity would produce only 20 sequential 4mer ligations(400×25% x 20%) after 400 cycles. Using these assumptions, a 400 cycleassay (1.5 days at 6′/cycle) would produce an average product length 80nucleotides.

In the promiscuous process (illustrated with Class I Xprobes in FIG.45), if Xprobe hybridization and ligation reactions are allowed to occurspontaneously and simultaneously throughout the DNA target template,replication of a much longer DNA template can be achieved using muchfewer cycles. Following each cycle of hybridization and wash, ligationcan be used to connect the remaining duplexes that are both adjacent and100% correct. Another, more stringent wash can be used to remove smallerligation products (everything 8mer and less; some 12mers) as well as allunligated 4mers. With ligation reactions occurring throughout the1,000-10,000 nucleotide target sequence, assay processivity goes updramatically. Since the promiscuous approach for Xprobe hybridizationand ligation is independent of the DNA target template length, themajority of the target template can be replicated in a fraction of thecycles required for the serial method. Longer cycle times can be used tocompensate for kinetic limitations if such an adjustment increases thefidelity and/or quantity of hybridization/ligation reactions.

FIG. 45 illustrates a sequential progression of the promiscuous process.Step I illustrates 4mer Xprobes hybridizing at multiple loci along theDNA target. Step II illustrates hybridization and ligation of adjacentXprobes; ligated Xprobes are stabilized and thus remain hybridized whilethe unligated Xprobes are melted off. Step III and Step IV illustrateanother thermal cycle of hybridization followed by ligation and thermalmelting of unligated Xprobes. Each cycle preferentially extends theexisting ligated Xprobe chains. As illustrated in Step IV, afterrepeated cycles, the DNA target is saturated with Xprobes leaving gaps,shorter than a probe length, where no duplexing has occurred. Tocomplete the Xpandomer the Xprobes are linked across the gaps byenzymatic or chemical means as illustrated in Step V.

Standard Gap Filling

As illustrated with Xprobes in Step V of FIG. 45, after completion ofthe hybridization and ligation cycling process, sequence gaps can befilled to produce a continuous Xpandomer. Gaps along the target DNAbackbone can be filled using well established DNA polymerase/ligasebased gap filling processes (Stewart et al., “A quantitative assay forassessing allelic proportions by iterative gap ligation”, Nucleic AcidsResearch 26(4):961-966, 1998). These gaps occur when adjacent chains ofXprobes meet each other and have a 1, 2 or 3 nucleotide gap lengthbetween them (assuming a 4mer Class 1 Xprobe as illustrated in FIG. 45).Gap filling can also be done via chemical cross linking (Burgin et al.).After gap filling is completed, the target DNA compliment, which iscomposed principally of ligated Xprobes with periodic 1, 2 or 3nucleotide fillers, can be processed (purification, cleavage, endmodification, reporter labeling) as appropriate to produce a measurableXpandomer. Since the SBX assay products are prepared and purified inbatch processing prior to the detection step, detection is efficient andnot rate limited by any concurrent biochemical processing.

In order to differentiate the gaps from nucleotide specific reportersignal, modified deoxynucleotide triphosphates (that are either alreadylabeled or capable of being labeled post assay) can be used to identifygap sequence. Suitable deoxynucleotide triphosphates are illustrated inFIGS. 46A (linker modified bases) and 46B (biotin modified bases).

Gap length and frequency are dependent upon a number of synthesisvariables, including cycle number, hybridization stringency, librarystrategy (shotgun vs. sub-library with stoichiometric adjustment),target template length (100 b-1 Mb), and reaction density (0.1-10 B).Gap frequency and gap length can be significantly reduced by utilizingmaximum stringency conditions, provided the conditions are compatiblewith the specified assay time range. Target length and reaction densityare also important factors with respect to both alignment fidelity andgap filling probability.

The promiscuous process of hybridization can use thermal cycling methodsto improve hybridization stringency and to increase the frequency ofadjacent substrate construct alignments. The hybridization, ligation andthermal melting continue under a precise thermal cycling routine untilthe majority of the target sequence is duplexed with probe.Weakly-bound, non-specific probe-target duplexes can be removed by asimple wash step, again under precise thermal control. Enzymatic orchemical ligation can be performed to link any adjacently hybridizedoligomeric constructs along the target DNA, generating longer and morestabilized sequences. Enzymatic ligation has the added benefit ofproviding an addition duplex fidelity crosscheck since mismatchedprobe-targets are not efficiently ligated. Using a second wash underprecise thermal cycling conditions, unlinked substrate constructs can bemelted off the DNA backbone; longer intermediates will remain. Thelonger ligated sequences grow at multiple loci along the target DNAuntil replication is mostly complete. Thehybridization/wash/ligation/wash process is repeated zero to many timesuntil the majority of target template has been replicated.

Temperature cycling coupled with substrate construct stabilization dueto base stacking can be utilized to enrich for adjacent hybridizationevents. Thermal cycling conditions, similar in concept to “touchdownPCR”, can be used to eliminate less stable, non-adjacent duplexes. Forexample, by repetitively cycling between the hybridization temperatureand the upper melting temperature (“Tm”) estimate for the oligo library(as determined by single probe melting—i.e., not base stack stabilized)adjacent probe hybridization events can be positively selected for. Withsufficient enrichment efficiency, the number of cycles can besignificantly reduced. This can be performed in single bath ormulti-bath conditions.

With a small library size (256, 4mer substrate constructs, for example),the library can be partitioned into sub-libraries on the basis of aneven narrower range of melting temperature. This strategy cansignificantly benefit the method if Tm bias cannot be controlled throughbase modification, hybridization solution adjuvants, and the like.Further, the probe stoichiometry can be adjusted (up or down) tocompensate for any residual biases that can still exist in thelibrary/sub-libraries.

Computer modeling was performed to evaluate statistics of gap occurrenceand the lengths of consecutively connected substrate constructs that arehybridized on a target DNA template. The model simulates ahybridization/ligation-thermal melt cycle with a complete library of256, 4mer Xprobes. The model results that are presented here are basedupon the following model process:

i) The hybridization/ligation step simulates random 4mer Xprobesrandomly meeting a randomly sequenced DNA template at random positionsand hybridizing if they match. No Xprobes can overlap any nucleotides.Hybridization continues until all locations on the target greater than 3nucleotides long are hybridized.

ii) The thermal melt step is simulated by removing all Xprobes that arein chains shorter than M 4mer Xprobes in length (where M=2, 3, 4, 5 . .. ). The longer chains remain on the DNA template. A “Chain” is definedhere as multiple consecutive Xprobes with no gaps between them.

iii) Repeat the cycle defined by i) and ii) so that Xprobes randomlybuild off of the existing loci of multi-Xprobe chains. The cycling stopswhen no change occurs between 2 consecutive cycles.

FIG. 47 and FIG. 48 each have two graphs that illustrate the statisticsof running the model 100 times on a random DNA template. For processingpurposes the DNA target was chosen to be 300 nucleotides long (but thestatistics discussed here are not changed if the template length is 5000nucleotides). FIG. 47 shows the results for M=3, where chains shorterthan M=3 get “melted off” the DNA template. FIG. 47A shows thestatistics of the chain lengths at the last cycle. The distributionappears to be quasi exponential ranging from 12mer (M=3) up to some rarelarge 160mers with a mean of ˜28 nucleotides in length. FIG. 47B showsthat a range of 5 to 20 cycles are needed to complete the DNA templateusing this method of chain “growth” and that 90% of the runs arecomplete after 12 cycles. FIGS. 48A and 48B show the same type ofinformation for the case where M=4, a more stringent thermal melt. Inthis case the average chain length increases to ˜40 nucleotides, butrequires ˜18 cycles to complete 90% of the runs. The longer chainlengths reduce the probability of having a gap as expected. The meanprobability of not filling a nucleotide position for this M=4 data has amean of 0.039 (approximately 1 in 25).

Gap Filling with 3mer and 2mer Substrate Constructs

An extension of the basic 4mer hybridization embodiment is to use 3merand 2mer substrate constructs to fill 3 base and 2 base sequence gaps,respectively. FIG. 49 shows an example of gap filling by sequential orsimultaneous addition of shorter Xprobes. If the majority of the targettemplate is successfully duplexed with Xprobes following 4merhybridization, most of the remaining gaps are either 1, 2 or 3 bases inlength. Average size of the Xpandomer can be increased significantly ifa moderate to high percentage of the 2 and 3 base gaps can be filled andligated with smaller Xprobes. Since hybridization of these smallerprobes is done after most of the target is duplexed, temperaturestringency can be reduced to allow for 3mer and then 2mer duplexing,respectively. Duplex stability of the smaller Xprobes is increased dueto base stacking stabilization. Following ligation, remaining gaps arefilled using a polymerase to insert nucleotides followed again byligation to link all remaining adjacent 5′ phosphates and 3′ hydroxyls.The polymerase incorporated linker modified nucleotides, allow for gaplabeling, either before or after incorporation. These labels can orcannot identify the base. On the basis of statistical models, summarizedin the FIG. 50 charts, the full length Xpandomer polymers have greaterthan 95% target sequence coverage (greater than 16 bases). According tothe model, the use of 3mer/2mer Xprobe gap filling drives down theunlabeled base percentage by a factor of 6 (assuming high efficiencyincorporation), which, for a 5000 base target sequence, would result in5000 bases of mostly contiguous sequence with less than 50un-discriminated single base gaps throughout the target replicon. Assuch, at 16× coverage greater than 99% of the unidentified gap sequencesis identified.

Gap filling with these smaller 3mer and 2mer Xprobes, for example,extends the average Xpandomer length by three-fold. The chartssummarizing the results of statistical modeling as shown in FIGS. 50Aand 50B indicate that the average length 3mer/2mer gap filledXpandomers, under the described conditions, are in the range of 130bases of sequence. Further, with 5000 gigabases of Xpandomer sequencesynthesized in this manner, which is the equivalent of converting only2.5 ng target sequence to Xpandomer, size selection of the longest 10%of Xpandomer fragments from that population would produce 500 gigabasesof sequence with average lengths in the range of 381 bases while sizeselection of the longest 2% of Xpandomer fragments would produce 100gigabases of sequence with average lengths in the range of 554 bases.These fragment lengths can be achieved without the need for single basegap filling with polymerase. If 4mer Xprobe hybridization is donewithout any gap filling, statistical models indicate that the averagelengths for the longest 10% and 2% of the fragments would be 106 and 148bases, respectively.

Addition of Adjuvants to Reduce Target Secondary Structure

Long, surface tethered single-stranded target DNA can present achallenging hybridization target due to intramolecular secondarystructures. The addition of an end tethered bead, as described in FIG.41E, reduces the occurrence of these intramolecular formations, anddestabilizes them when they do occur. However, it can be found necessaryto further diminish secondary structure formation, which can effectivelyblock substrate construct hybridization, with the addition of adjuvants.

The addition of unlabeled 2mer and/or 3mer oligonucleotide probes to,for example, an Xprobe hybridization mix can serve this purpose. Toeliminate the possibility of incorporation into the Xpandomer, theseprobes are synthesized to be un-ligatable (no 5′ phosphate or 3′hydroxyl). Due to their small size, the 2mer and/or 3mer adjuvants arenot likely to form stable duplexes at the 4mer Xprobe hybridizationtemperatures; however, their presence in the reaction mix at moderate tohigh concentrations reduces the frequency and stability of targetsecondary structure by weakly and transiently blocking access ofintramolecular nucleotide sequences that can otherwise duplex. FIG. 51is an illustration of how the addition of 2mer or 3mer adjuvants inhibitsecondary structure formation (for simplicity only, 4mer Xprobes werenot shown in the figure). Coupled with the elongational force providedby the bead tether, adjuvants significantly diminish the frequency ofsecondary structure formation.

These dinucleotide and/or trinucleotide adjuvants can be composed ofstandard nucleotides, modified nucleotides, universal nucleotides(5-nitroindole, 3-nitropyrrole and deoxyinosine), or any combinationthereof to create all necessary sequence combinations. FIG. 52 showscommon universal base substitutions that used for this purpose.

Another approach for reducing target secondary structure is to replicatetarget DNA to produce a synthetic cDNA target that has a reducedsecondary structure stability as compared to native DNA. FIG. 53 listssome nucleotide analogs, as illustrated in US application 2005/0032053(“Nucleic acid molecules with reduced secondary structure”), that can beincorporated into the described synthetic cDNA target. These analogs,which include (but are not limited to) N4-ethyldeoxycytidine,2-aminoadenosine-5′-monophosphate, 2-thiouridine-mono phosphate,inosine-monophosphate, pyrrolopyrimidine-monophosphate and2-thiocytidine-mono phosphate, have been demonstrated to reduce cDNAsecondary structure. This approach can be used along with targetelongation and hybridization adjuvants to reduce secondary structure.

Detection and Measurement

As mentioned previously, the Xpandomer can be labeled and measured byany number of techniques. The massive data output potential of the SBXmethod is well matched to sensor arrays based on nanopores or equivalenttechnologies. In one embodiment, the nanopore array can function as anionization source on the front end of a mass spectrometer, wherein thereporter codes on the Xpandomer are cleavable mass spectroscopy tags.Other embodiments involve use of a nanopore sensor such as electricalimpedance/conductance or FRET.

One detection embodiment uses a nanopore array as depicted in FIG. 54.In this embodiment, the Xpandomer is assembled using methods describedpreviously, except that the DNA target is not anchored to an immobilizedsolid support but is anchored to a magnetic bead that has its anchoringprobe threaded through a nanopore substrate. Furthermore, a multitude ofFRET donor fluorophores, for two excitation wavelengths, are tethered tothe nanopore entrance (shown as small squares). The FRET acceptorfluorophores constitute the reporters incorporated on the Xpandomer.After the linked Xprobes of the Xpandomer are cleaved, the Xpandomer isstretched by a force that can be electrostatic, magnetic, gravitationaland/or mechanical and can be facilitated by the bead used to extend theoriginal DNA target (shown attached to the top of the Xpandomer). In thefinal measurement step the Xpandomer is drawn through the nanopore byapplying a magnetic force on the magnetic bead shown below the nanoporestructure. The donor fluorophores at the nanopore entrance are excitedwith a light source (λ₁) and as the reporters pass proximal to the donorfluorophores, they are excited and emit their signature fluorescence(λ₂) that decodes to the associated nucleotide sequence.

FIG. 54 also illustrates one embodiment of encoding the sequence in thereporter codes. On each tether, there are 4 reporter sites, each loadedwith a combination of two FRET acceptor fluorophore types. This providesfour measurable states on each site using relative intensity level. Ifthe acceptor fluorophores' fluorescence were red and green, the encodingof these states to the 4 nucleotides can be: A: green>red, C: only red,G: only green and T: red>green.

In another embodiment, the Xpandomer is labeled with mass tag reportersthat are measured using a mass spectrometer. The Xpandomer is meteredinto a narrow capillary which feeds the reporters sequentially into anelectrospray ionizer. To enable mass spectrometer measurement of thediscrete mass tag reporters, the mass tags may be photocleaved from thereporter scaffolding just prior to, during, or after mass tagionization. Magnetic sector, quadrupole, or time of flight (“TOF”) basedmass spectrometer can be used for mass tag detection. The instrumentonly requires distinguishing a limited number of mass-spaced tags. Thiscan be used to improve sensitivity and throughput of the instrument.Also, employing an instrument having multiple channels for performing ofionization and detection in parallel increases throughput by orders ofmagnitude.

One embodiment of mass spectrometer approach detection is amulti-channel TOF mass detector that can read >100 channelssimultaneously. A suitable instrument would use a multi-channelionization source that feeds Xpandomer into multiple channels at aconcentration and rate that maximizes the channel usage, thus maximizingthe output rate of good quality data. Such an ionization source requireshaving adequate separation of the mass tag reporter segments. Mass tagscan be photocleaved as they emerge from a nanopore and are ionized. Thedispersion requirements are not high so that a short flight tube is allthat is required. Extremely high measurement output is possible with amulti-channel mass spectrometer detection approach. For example, anarray of 100 nanopore ion channels reading at the rate of 10,000reporter codes per second (with 4 nucleotide measurements per reportercode) would achieve instrument throughput of >4 Mbases/second.

In another embodiment, the nanopore is used in a manner similar to aCoulter counter (FIG. 55). In this implementation, the charge density ofthe Xpandomer is designed to be similar to that of native DNA. Thereporters are designed to produce 3 levels of impedance as measured inthe nanopore detector, for example. The Xpandomers are presented in afree solution that has high concentrations of electrolyte, such as 1MKCl. The nanopore is 2 to 15 nm in diameter and is 4 to 30 nm long. Toachieve good resolution of the tether constructs labels they are chosento be close to the same length or longer. The diameter of the labels canbe different for the 3 levels. The Xpandomer segment near the tetherconstruct linkages have no reporters (for example PEG), this segmentwill have a particular impedance level. One of the three reporter levelscan be equivalent. Different impedance signatures, which involve bothlevel of impedance and temporal response, can be produced by varyingsegment lengths, charge density, and molecular density. For example, toachieve 3 different impedance levels, labeled tether constructs segmentscan be chemically coded to couple to one of three different polymertypes, each with a different length and charge density.

When the Xpandomer passes through the nanopore detector the current ismodulated according to which label type is present. The amount ofcharged polymer residing in the nanopore affects both the electrolytespecies current and the translocation velocity.

Polymer-based detection by nanopores is demonstrated in U.S. Pat. Nos.6,465,193 and 7,060,507, for example, and the physical parameters of apolymer are shown as expected to modulate electrical output from ananopore. These patents and related arts are herein incorporated in fullby reference.

In another embodiment of a nanopore-based detection apparatus (FIG. 56)lateral electrodes affixed to the nanopore are used to measure impedanceor conductivity from side-to-side in the nanopore while a voltage isapplied across the solid support film. This has an advantage ofseparating the translocation function from the impedance function. Asthe Xpandomer is conveyed through the nanopore, current modulation isagain measured. Microfluidic and micropipetting techniques are employed,along with drag tags, magnetic beads, electrophoretic stretchingtechniques, and so forth, in order to convey the Xpandomer through thenanopore. For example, end-labeled free-solution electrophoresis, alsotermed ELFSE, is a method for breaking the charge to friction balance offree-draining DNA that can be used for free-solution Xpandomerelectrophoresis (Slater et al., “End-labeled free-solutionelectrophoresis of DNA”, Electrophoresis 26: 331-350, 2005).

Methods for tethering, stretching, labeling, and measuring large DNAfragments are well established (Schwartz et al., “A single-moleculebarcoding system using nanoslits for DNA analysis”, PNAS,104(8):2673-2678, 2007; and Blanch et al., “Electrokinetic Stretching ofTethered DNA”, Biophysical Journal 85: 2539-2546, 2003). However, singlenucleobase resolution for the purposes of whole genome sequencing ofnative nucleic acids is beyond the capabilities of these techniques. InFIG. 57 through 59, several “single-molecule” Xpandomer detectionmethods are pictured. In FIG. 57, a microscope is pictured and any of avariety of direct imaging techniques that take advantage of the greaterspatial resolution of the Xpandomer structure can be conceived. AnXpandomer molecule is placed flat on a generally planar surface andscanned along its length. Examples include light microscopy andfluorescence microscopy. Using high resolution microscopy is limited inmost cases to >100 nm resolution which is still possible with longtethers (100 nm per reporter for example). Higher resolution (requiringtethers<100 nm per reporter for example) can be achieved usingtechniques such as total internal reflection fluorescence microscopy(TIRF: Starr, T. E. et. al., Biophys. J. 80, 1575-84, 2001) zero-modewaveguides (Levine M. J. et al., Science 299, 682-85, 2003) or nearfield optical microscopy (NSOM: de Lang et al., J. Cell Sci. 114,4153-60 2001) or Confocal laser scanning microscopy. In the case oflocalized FRET excitation interactions can be localized to <10 nmleading to tether lengths of ˜10 nm/reporter.

Detection and analysis of large DNA molecules by electron microscopy iswell established (Montoliu et al., “Visualization of large DNA moleculesby electron microscopy with polyamines: application to the analysis ofyeast endogenous and artificial chromosomes”, J. Mol. Bio.246(4):486-92, 1995), however, accurate and high-throughput sequencingof polynucleotides using these methods have proven difficult. In FIG.58, transmission (TEM) and scanning electron microscopy (SEM) areindicated conceptually for detection of an Xpandomer. Here a focusedelectron beam is used to scan an Xpandomer, which is again generallyflat on a surface. Aspects of the structure of the Xpandomer serve todecode the genetic information on the backbone. Specimen fixation andsputter coating techniques, which enable imaging of individual andatom-sized features of molecules, can be used to enhance magnification.

Also of interest are nanoelectrode-gated electron tunneling conductancespectroscopy, in which a tunneling electron beam between twonanoelectrode tips is modulated by conveyance of the Xpandomer betweenthe tips (Lee et al., “Nanoelectrode-Gated Detection of IndividualMolecules with Potential for Rapid DNA Sequencing”, Solid StatePhenomena 121-123: 1379-1386, 2007). The Xpandomer perturbs thetunneling current by its screening-conduction effect, which can beamplified over native DNA by use of suitable reporters. This techniquehas the advantage that specimen fixation and the requirement for vacuumis avoided, and in theory, massively parallel arrays of electrode gatescan be employed to read many Xpandomers in parallel.

In FIG. 59, atomic force microscopy is illustrated conceptually. In asimple embodiment, a nanotube mounted on a sensitive cantilever sweptacross a surface and the attractive and repulsive forces between theprobe and the sample surface are translated into a topological pictureof the surface being scanned. This technique can achieve very highresolution but has relatively slow scan speeds (M. Miles, Science 277,1845-1847 (1997)) Scanning tunneling electron microscopy (STM) is arelated technology for imaging surfaces; the probe however does nottouch the surface but rather a tunneling current between the surface andthe probe is measured. Here the Xpandomer can be laid flat on a surfaceand physically scanned with the probe tip, much like a phonograph needleon a record.

Sequence Assembly

The published human genome reference sequence (or other referencesequence) can be used as an alignment tool to assist assembling themassive amounts of sequence data produced with SBX. Despite the likelyinclusion of small, positionally identified sequence gaps, the long readlength capabilities described for Xpandomer-based SBX simplifies andimproves the fidelity of assembling whole genome sequences. As discussedabove, the process can be further simplified by partitioning contiguousfragments in dimensionally confined locations on the assay reactionsurface. In this embodiment the parsing method can dramatically reduceassembly time and error.

Monomeric Constructs

FIG. 9 provides an overview of monomeric constructs of the invention. Atotal of five classes are distinguished including four RT-NTP classes(VI, VII, VIII, and IX) and one XNTP class (X). Each class will bediscussed individually below.

Monomeric constructs of Classes VI to X are distinguished from Class Ito V oligomeric constructs in that they use a single nucleobase residueas a substrate. In the following description, N, refers to anynucleobase residue but is typically a nucleotide triphosphate or analogherein. It has attachment points on a tether (also described herein) forexample, to the heterocyclic rings of the base, to the ribose group, orto the α-phosphate of the nucleobase residue. As described, the primarymethod for template-directed synthesis uses polymerase but any methodthat can perform template-directed synthesis is appropriate includingmethods of chemical and enzymatic ligation.

For the substrate constructs where and δ linker groups are used tocreate inter-subunit linkages, a broad range of suitable commerciallyavailable chemistries (Pierce, Thermo Fisher Scientific, USA) can beadapted for this purpose. Common linker chemistries include, forexample, NHS-esters with amines, maleimides with sulfhydryls,imidoesters with amines, EDC with carboxyls for reactions with amines,pyridyl disulfides with sulfhydryls, and the like. Other embodimentsinvolve the use of functional groups like hydrazide (HZ) and4-formylbenzoate (4FB) which can then be further reacted to formlinkages. More specifically, a wide range of crosslinkers (hetero- andhomo-bifunctional) are broadly available (Pierce) which include, but arenot limited to, Sulfo-SMCC (Sulfosuccinimidyl4-[N-maleimidomethyl]cyclohexane-1-carboxylate), SIA (N-Succinimidyliodoacetate), Sulfo-EMCS ([N-e-Maleimidocaproyloxy]sulfosuccinimideester), Sulfo-GMBS (N-[g-Maleimido butyryloxy]sulfosuccinimide ester),AMAS N-(a-Maleimidoacetoxy) succinimide ester), BMPS (N EMCA(N-e-Maleimidocaproic acid)-[β-Maleimidopropyloxy]succinimide ester),EDC (1-Ethyl-3-[3-dimethylaminopropyl]carbodiimide Hydrochloride),SANPAH (N-Succinimidyl-6-[4′-azido-2′-nitrophenylamino]hexanoate), SADP(N-Succinimidyl(4-azidophenyl)-1,3′-dithiopropionate), PMPI(N-[p-Maleimidophenyl]isocy, BMPH (N-[β-Maleimidopropionicacid]hydrazide, trifluoroacetic acid salt) anate), EMCH([N-e-Maleimidocaproic acid]hydrazide, trifluoroacetic acid salt), SANH(succinimidyl 4-hydrazinonicotinate acetone hydrazone), SHTH(succinimidyl 4-hydrazidoterephthalate hydrochloride), and C6-SFB(C6-succinimidyl 4-formylbenzoate). Also, the method disclosed byLetsinger et al. (“Phosphorothioate oligonucleotides having modifiedinternucleoside linkages”, U.S. Pat. No. 6,242,589) can be adapted toform phosphorothiolate linkages.

Further, well established protection/deprotection chemistries arebroadly available for common linker moieties (Benoiton, “Chemistry ofPeptide Synthesis”, CRC Press, 2005). Amino protection include, but arenot limited to, 9-Fluorenylmethyl carbamate (Fmoc-NRR′), t-Butylcarbamate (Boc-NRR′), Benzyl carbamate (Z-NRR′, Cbz-NRR′), AcetamideTrifluoroacetamide, Phthalimide, Benzylamine (Bn-NRR′),Triphenylmethylamine (Tr-NRR′), and Benzylideneaminep-Toluenesulfonamide (Ts-NRR′). Carboxyl protection include, but are notlimited to, Methyl ester, t-Butyl ester, Benzyl ester, S-t-Butyl ester,and 2-Alkyl-1,3-oxazoline. Carbonyl include, but are not limited to,Dimethyl acetal 1,3-Dioxane, and 1,3-Dithiane N,N-Dimethylhydrazone.Hydroxyl protection include, but are not limited to, Methoxymethyl ether(MOM-OR), Tetrahydropyranyl ether (THP-OR), t-Butyl ether, Allyl ether,Benzyl ether (Bn-OR), t-Butyldimethylsilyl ether (TBDMS-OR),t-Butyldiphenylsilyl ether (TBDPS-OR), Acetic acid ester, Pivalic acidester, and Benzoic acid ester.

Herein, the tether is often depicted as a reporter construct with threereporter groups, various reporter configurations can be arrayed on thetether, and can comprise a single reporter to identify the substrate,multiple reporters to identify the substrate, or the tether may be nakedpolymer (having no reporters). Note that reporters may be used fordetection synchronization, error correction, redundancy, or otherfunctions. In the case of the naked polymer, the reporters may be thesubstrate itself, or may be on a second tether attached to thesubstrate. In some cases, one or more reporter precursors are arrayed onthe tether, and reporters are affinity bound or covalently boundfollowing assembly of the Xpandomer product.

Reporter coding strategies are disclosed above, and further discussedbelow. For example, two bit, binary coding of each monomer would producefour unique code sequences (11, 10, 01, 00) which can be used toidentify each sequence base (adenine “A”, cytosine “C”, guanine “G”,thymidine “T”), assuming substrate coupling is directional. Ifnon-direction, then a third bit provides unambiguous coding.Alternatively, a single 4-state multiplexed reporter construct providesa unique reporter code for each sequence base. A variety offunctionalization and labeling strategies can be considered for tetherconstructs, including for examples functionalized dendrimers, polymers,branched polymers, nanoparticles, and nanocrystals as part of a reporterscaffold, as well as reporter chemistries with a detectioncharacteristic to be detected with the appropriate detection technologyincluding, for example, fluorescence, FRET emitters or exciters, chargedensity, size or length. Base specific labels can be incorporated intothe tether as labeled substrates either prior to or after assembly ofthe Xpandomer. Once the Xpandomer is fully released and elongated, thereporter codes can be detected and analyzed using a range of detectionmethods.

Libraries of substrates suitable as monomeric substrates include (butare not limited to) modified ATP, GTP, CTP, TTP and UTP.

Class VI Monomeric Constructs

FIG. 60 describes Class VI monomeric substrate constructs (a type ofRT-NTP) in more detail. FIGS. 60A through 60C are read from left toright, showing first the monomeric substrate construct (Xpandomerprecursor having a single nucleobase residue), then the intermediateduplex daughter strand in the center, and on the right the Xpandomerproduct prepared for sequencing.

As shown in FIG. 60A, the monomeric substrate constructs of Class VIhave a tether, T (600), attached by a linkage (601) of a first endmoiety to a substrate nucleobase residue, N. A linker group, ε, isdisposed on the first end moiety (603) of the tether proximate to R¹. Atthe distal end (604) of the tether, a second end moiety with a secondlinker group, δ, is positioned proximate to R². The second end moiety ofthe tether is secured to the first end moiety in proximity to thenucleobase by an intra-tether selectively cleavable crosslink (or byother constraint). The intra-tether cleavable crosslink (605) is denotedhere by dotted line, which can indicate, for example, a disulfide bondor a photocleavable linker. This constraint prevents the tether fromelongating or expanding and is said to be in its “constrainedconfiguration”. Under template-directed assembly, substrates form aduplex with the target template such that the substrates are abutted.Under controlled conditions, collocated linker groups δ and ε of theabutting substrates link to form a bond between the adjacent substrateconstructs. Linker groups δ and ε of a monomeric substrate construct donot form an intra-substrate bond due to positioning constraints.Suitable linkage and protection/deprotection chemistries for δ, ε, and χare detailed in the general monomeric construct description.

R¹ and R² are end groups configured as appropriate for the synthesisprotocol in which the substrate construct is used. For example,R¹=5′-phosphate and R²=3′-OH, would find use in a ligation protocol, andR¹=5′-triphosphate and R²=3′-OH for a polymerase protocol. Optionally,R² can be configured with a reversible blocking group for cyclicalsingle-substrate addition. Alternatively, R¹ and R² can be configuredwith linker end groups for chemical coupling or with no linker groupsfor a hybridization only protocol. R¹ and R² can be of the general typeXR, wherein X is a linking group and R is a functional group.

During assembly, the monomeric substrate construct is first polymerizedon the extendable terminus of the nascent daughter strand by a processof template-directed polymerization using a single-stranded template asa guide. Generally, this process is initiated from a primer and proceedsin the 5′ to 3′ direction. Generally, a DNA polymerase or otherpolymerase is used to form the daughter strand, and conditions areselected so that a complementary copy of the template strand isobtained. Subsequently, linker group δ, which is now collocated with thelinker group ε of the adjacent subunit tether, are selectivelycrosslinked to form a χ-bond, which is an inter-tether, inter-subunitbond. The χ-bonds join the tethers in a continuous chain, forming anintermediate termed the “duplex daughter strand”, as shown in FIG. 60B.After the χ-bond is formed, the intra-tether bond may be broken.

The duplex daughter strand (FIG. 60B) is a hetero-copolymer withsubunits shown in brackets. The primary backbone (˜N˜)κ, template strand(—N′-)κ, and tether (T) are shown as a duplexed daughter strand, where Kdenotes a plurality of repeating subunits. Each subunit of the daughterstrand is a repeating “motif” and the motifs have species-specificvariability, indicated here by the α superscript. The daughter strand isformed from monomeric substrate construct species selected by atemplate-directed process from a library of motif species, the monomersubstrate of each substrate construct species binding to a correspondingcomplementary nucleotide on the target template strand. In this way, thesequence of nucleobase residues (i.e., primary backbone) of the daughterstrand is a contiguous, complementary copy of the target templatestrand.

Each tilde (˜) denotes a selectively cleavable bond shown here as theinter-substrate bonds. These are selectively cleavable to release andexpand the tethers (and the Xpandomer) without degrading the Xpandomeritself.

The daughter strand is composed of an Xpandomer precursor called the“constrained Xpandomer” which is further composed of tethers in the“constrained configuration”. When the tethers convert to their “expandedconfiguration”, the constrained Xpandomer converts to the Xpandomerproduct. The tethers are constrained by the inter-subunit χ linkages,the substrate attachment and, optionally, the intra-tether linkages ifstill present. The χ linkage attaches the tether first end moiety of afirst subunit to the tether second end moiety at the abutting end of asecond subunit and is formed by linking the collocated linker groups, εof the first subunit, and δ of the second subunit.

The daughter strand can be seen to have two backbones, a “primarybackbone”, and the backbone of the “constrained Xpandomer”. The primarybackbone is composed of the contiguously abutted and polymerizedmonomeric substrates. The “constrained Xpandomer backbone” bypasses theselectively cleavable linkage between the monomer substrates and isformed by χ bond-linked backbone moieties, each backbone moiety being atether. It can be seen that the constrained Xpandomer backbone bridgesover the selectively cleavable bonds of the primary backbone, and willremain covalently intact when these selectively cleavable bonds arecleaved and the primary backbone is fragmented.

The tether χ bond (crosslinking of linker groups δ and ε) is generallypreceded by enzymatic coupling of the monomer substrates to form theprimary backbone, with, for example, phosphodiester bonds betweenadjacent bases. In the structure shown here, the daughter strand primarybackbone has been formed, and the inter-substrate, are depicted by atilde (˜) to indicate that they are selectively cleavable. Afterdissociating or degrading the target template strand, cleaving theselectively cleavable bonds (which include the intra-tether bonds), theconstrained Xpandomer is released and becomes the Xpandomer product.Methods for dissociation of the template strand include heatdenaturation, or selective digestion with a nuclease, or chemicaldegradation. A method for selectable cleavage uses nuclease digestionwhere for example, phosphodiester bonds of the primary backbone aredigested by a nuclease and tether-to-tether bonds are nucleaseresistant.

FIG. 60C is a representation of the Class VI Xpandomer product afterdissociation of the template strand and after cleavage of theselectively cleavable bonds (including those in the primary backboneand, if not already cleaved, the intra-tether links). The Xpandomerproduct strand contains a plurality of subunits κ, where κ denotes theκth subunit in a chain of m subunits making up the daughter strand,where κ=1, 2, 3 to m, where m>10, generally m>50, and typically m>500or >5,000. Each subunit is formed of a tether in its expandedconfiguration and is stretched to its length between the χ linkages ofadjacent subunits. The pendant substrate is attached to the tether ineach subunit. Each subunit, a subunit motif α, contains species-specificgenetic information established by template-directed assembly of theXpandomer intermediate (daughter strand).

FIG. 60D shows the substrate construct of FIG. 60A as a molecular model,where the monomer substrate member, represented with a nucleobaseresidue (606), is joined to the tether by a linkage (607) of the tetherfirst end moiety. Also disposed on the first end moiety is a linkergroup (608), shown as ε in FIG. 60A. A second linker group (609), shownas δ in FIG. 60A, is disposed on the second end moiety at the distal endof the tether. A selectively cleavable intra-tether linkage (602),depicted by the adjoining triangles, is shown that constrains the tetherby linking the first and second end moiety. The linker groups ε and δare positioned to not interact and to preferably align near the R¹ andR² sides of the substrate, respectively. The tether loop (600) shownhere has three reporters (590,591,592), which can also be motif speciesspecific.

FIG. 60E shows the substrate construct after incorporation into theproduct Xpandomer. The subunits are cleaved and expanded and are linkedby χ bonds (580,581), depicted as an open oval, formed by linking thelinker groups δ and ε referred to in FIG. 60A. A subunit is indicated bydotted lines vertically bracketing the repeating subunit, as representedby brackets in the accompanying FIG. 60C.

In the Xpandomer product of FIG. 60E, the primary backbone has beenfragmented and is not covalently contiguous because any direct bondbetween the substrates of adjacent subunits has been cleaved. Throughthe cleavage process, the constrained Xpandomer is released to becomethe Xpandomer product. The tether members that were formerly inconstrained configuration are now in expanded configuration, therebyfunctioning to linearly stretch out the sequence information of thetemplate target. Expanding the tethers lowers the linear density of thesequence information along the Xpandomer and provides a platform forincreasing the size and abundance of reporters which in turn improvessignal to noise for detection and decoding of the template sequence.

While the tether is depicted as a reporter construct with three reportergroups, various reporter configurations can be arrayed on the tether,and can comprise single reporters that identify monomer or the tethercan be a naked polymer. In some cases, one or more reporter precursorsare arrayed on the tether, and reporters are affinity bound orcovalently bound following assembly of the Xpandomer product.

Turning now to FIG. 61, a method for single base extension (SBE) withmonomeric substrate constructs is shown. End-adapted target templates(or random target template sequences, depending on the nature of theimmobilized primers), are first annealed to the immobilized primers.Before Step I, the immobilized templates (611) are contacted with amonomeric substrate construct library, a member of which is shown forillustration (612), and polymerase (P). Conditions are adjusted fortemplate-directed polymerization. In this example, the 5′ termini of thefirst monomeric substrate (R² of FIGS. 60A and 60D) is polymerized toinitiate the nascent daughter strand. The substrate has been substitutedat the 3′ termini (R¹ of FIGS. 60A and 60D), to reversibly block furtherextension. This is shown in more detail in magnified FIG. 61 a (dottedlines), where the monomeric substrate is shown abutting the primer. Thissubstrate is a 5′-triphosphate, and a phosphodiester bond is formed withthe primer by action of the polymerase. It should be noted that thereactive functional group (6 of FIG. 60A) shown in FIG. 61 c istypically deactivated (by reaction or other means) in the firstsubstrate added (as shown in FIG. 61 a by the rectangle).

In step I of FIG. 61, the blocking group is removed (free rectangle) toenable the addition of another monomer (extension). Methods forreversibly blocking the 3′ end include the use of Pd to catalyze removalof an allyl group to regenerate a viable 3′ hydroxyl end, or the use of3′-O-(2-Nitrobenzyl) terminated nucleotides, where active hydroxyls canbe regenerated by exposure to a UV source to photocleave the terminatingmoiety as described by Ju et al. (“Four-color DNA sequencing bysynthesis using cleavable fluorescent nucleotide reversibleterminators”, PNAS 26; 103(52): 19635-19640, 2006, and “Four-color DNAsequencing by synthesis on a chip using photocleavable fluorescentnucleotides”, PNAS 26; 102(17):5926-31, 2005). The regenerated 3′-OH isshown in more detail in magnified view FIG. 61 b. In this view, theabutting ends of the adjacent substrate construct and primer arepolymerized and the 3′OH end of the nascent daughter strand is activatedby removing the blocking group. In step II, the template is contactedwith a monomeric substrate construct library, where the functional δgroups are reactive under controlled conditions, and a monomer substrateis polymerized to the nascent daughter strand. This polymerizationcollocates the ε (608) and δ (609) groups of the abutted substrateconstructs as shown in FIG. 61 c. In step III, under controlledconditions, these groups react to form a χ bond (580), as shown in moredetail in FIG. 61 d.

As indicated, cycling through steps II, III and IV extends the nascentdaughter each time by additional substrate (construct). Typically a washstep may be used to remove unreacted reagents between steps. The processis thus analogous to what is termed in the literature, cyclical “singlebase extension”. The process is shown with polymerase, P, but may beadapted for a ligase or a chemical ligation protocol suitable forjoining substrate constructs in a template-directed synthesis. Step Vshows the daughter strand intermediate for the Xpandomer (605). Thisintermediate can be dissociated from the template and the primer, forexample, with a nuclease that attacks the primary backbone of thedaughter strand, thereby relieving the constrained tethers and releasingthe Xpandomer product.

As with all SBE methods, efficient washing in between cycles is helpfulfor reducing undesirable side reactions. To further facilitate singlebase incorporation through template regions with high secondarystructure, the extension temperature can be varied throughout eachextension cycle and/or additives or adjuvants, such as betaine, TMACL,or PEG, can also be added to neutralize the effects of secondarystructure (as is done in conventional polymerase extension protocols).And if necessary, stoichiometry of the substrate construct species canbe varied to compensate for reaction bias favoring certain bases, suchas C or G.

An alternative method for producing Class VI Xpandomers is to dopolymerase-based processive polymerization. DNA and RNA polymerases, aswell as any similarly functioning enzymes demonstrated to catalyzeaccurate polymerization of RT-NTPs, absent reversibly terminal blockingR groups, can be considered for this approach.

A broad range of crosslinking chemistries are known in this field, andare useful for formation of χ bonds. These include use of NHS-esterswith amines, maleimides with sulfhydryls, imidoesters with amines, EDCwith sulfhydryls and carboxyls for reactions with amines, pyridyldisulfides with sulfhydryls, and so forth. Other embodiments involve theuse of functional groups like hydrazide (HZ) and 4-formylbenzoate (4FB)which can then be further reacted to link subunits. In one option, twodifferent linking chemistries, ε1/δ1 and ε2/δ2 (also referred to asL1/L1′ and L2/L2′ in other parts of this document) that react to form χ1and χ2 bonds respectively, and can be used to differentiallyfunctionalize two sets of RT-NTP substrate constructs. For example, ifone cycle of SBE is performed with a ε1/δ2 functionalized RT-NTP set,the following SBE cycle would use the ε82/δ1 set resulting in a reactionof the collocated δ2/ε2 pair to form χ1. Ordered activation of thecrosslinking pairs is useful for minimizing crosslinking errors andundesired side reactions.

Class VII Monomeric Constructs

Class VII molecules are analogs of Class VI described previously. Theprimary difference is that the cleavable bond is between the tether andthe substrate instead of between the substrates. In FIG. 62, Class VIImonomeric substrate constructs (a type of RT-NTP) are disclosed in moredetail. FIGS. 62A through 62C are read from left to right, showing firstthe monomeric substrate construct (Xpandomer precursor having a singlenucleobase residue), then the intermediate duplex daughter strand in thecenter, and on the right the Xpandomer product prepared for sequencing.

As shown in FIG. 62A, the monomeric substrate constructs of Class VIIhave a tether, T (620), attached by a selectively cleavable linker (621)to a first end moiety to a substrate nucleobase residue, N. Anotherlinker group, ε (622), is disposed on the first end moiety of the tetherproximate to R¹. At the distal end of the tether T a second end moietywith a second linker group, δ (623), is positioned proximate to R². Thesecond end moiety of the tether is secured to the first end moiety inproximity to the nucleobase by an intra-tether selectively cleavablecrosslink (624) (or by other constraint). The intra-tether cleavablecrosslink is denoted here by dotted line, which can indicate, forexample, a disulfide bond or a photocleavable linker. This constraintprevents the tether from elongating or expanding and is in a“constrained configuration”. Under template-directed assembly,substrates form a duplex with the target template such that thesubstrates are abutted. Under controlled conditions, collocated linkergroups δ and ε of the abutting substrates link to form a bond betweenthe adjacent substrate constructs. Linker groups δ and ε of a monomericsubstrate construct do not form an intra-substrate bond due topositioning constraints. Suitable linkage and protection/deprotectionchemistries for δ, ε, and χ are detailed in the general monomericconstruct description.

R¹ and R² are end groups configured as appropriate for the synthesisprotocol in which the substrate construct is used. For example,R¹=5′-phosphate and R²=3′-OH, would find use in a ligation protocol, andR¹=5′-triphosphate and R²=3′-OH for a polymerase protocol. Optionally,R² can be configured with a reversible blocking group for cyclicalsingle-substrate addition. Alternatively, R¹ and R² can be configuredwith linker end groups for chemical coupling or with no linker groupsfor a hybridization only protocol. R¹ and R² can be of the general typeXR, wherein X is a linking group and R is a functional group.

During assembly, the monomeric substrate construct is first polymerizedon the extendable terminus of the nascent daughter strand by a processof template-directed polymerization using a single-stranded template asa guide. Generally, this process is initiated from a primer and proceedsin the 5′ to 3′ direction. Generally, a DNA polymerase or otherpolymerase is used to form the daughter strand, and conditions areselected so that a complementary copy of the template strand isobtained. Subsequently, linker group δ, which is now collocated with thelinker group of the adjacent subunit tether, is caused to crosslink andforms a x-bond, which is an inter-tether, inter-subunit bond. Thex-bonds join the tethers in a continuous chain, forming an intermediatetermed the “duplex daughter strand”, as shown in FIG. 62B. After thex-bond is formed, the intra-tether bond may be broken.

The duplex daughter strand (FIG. 62B) is a hetero-copolymer withsubunits shown in brackets. The primary backbone (—N—)κ, template strand(—N′-)κ, and Tether (T) are shown as a duplexed daughter strand, where κdenotes a plurality of repeating subunits. Each subunit of the daughterstrand is a repeating “motif” and the motifs have species-specificvariability, indicated here by the a superscript. The daughter strand isformed from monomeric substrate construct species selected by atemplate-directed process from a library of motif species, the monomersubstrate of each substrate construct species binding to a correspondingcomplementary nucleotide on the target template strand. In this way, thesequence of nucleobase residues (i.e., primary backbone) of the daughterstrand is a contiguous, complementary copy of the target templatestrand.

The tilde (˜) denotes a selectively cleavable bond shown here as thetether to substrate linker. These are selectively cleavable to releaseand expand the tethers (and the Xpandomer) without degrading theXpandomer itself.

The daughter strand is composed of an Xpandomer precursor called the“constrained Xpandomer” which is further composed of tethers in the“constrained configuration”. When the tethers convert to their “expandedconfiguration”, the constrained Xpandomer converts to the Xpandomerproduct. The tethers are constrained by the inter-subunit χ linkages,the cleavable linkage to the substrate and, optionally, the intra-tetherlinkages if still present. The χ linkage attaches the tether first endmoiety of a first subunit to the tether second end moiety at theabutting end of a second subunit and is formed by linking the collocatedlinker groups, ε of the first subunit, and δ of the second subunit.

The daughter strand can be seen to have two backbones, a “primarybackbone”, and a “constrained Xpandomer backbone”. The primary backboneis composed of the contiguously abutted and polymerized monomericsubstrates. The constrained Xpandomer backbone bypasses the selectivelycleavable linkage that connects to the substrate and is formed by χ bondlinked backbone moieties, each backbone moiety being a tether. It can beseen that the constrained Xpandomer backbone bridges over theselectively cleavable bonds connected to the primary backbone, and willremain covalently intact when these selectively cleavable bonds arecleaved and the primary backbone is dissociated or fragmented.

The tether χ bond (crosslinking of linker groups δ and ε) is generallypreceded by enzymatic coupling of the monomer substrates to form theprimary backbone with, for example, phosphodiester bonds betweenadjacent bases. In the structure shown here, the daughter strand primarybackbone has been formed, and the inter-substrate, are depicted by atilde (˜) to indicate that they are selectively cleavable. Afterdissociating or degrading the target template strand, cleaving theselectively cleavable bonds (which include the intra-tether bonds), theconstrained Xpandomer is released and becomes the Xpandomer product.Methods for dissociation of the template strand include heatdenaturation, or selective digestion with a nuclease, or chemicaldegradation. A method for selectable cleavage uses nuclease digestionwhere for example, phosphodiester bonds of the primary backbone aredigested by a nuclease and tether-to-tether bonds are nucleaseresistant.

FIG. 62C is a representation of the Class VII Xpandomer product afterdissociation of the template strand and after cleavage of theselectively cleavable bonds (including those attached to the primarybackbone and, if not already cleaved, the intra-tether links). TheXpandomer product strand contains a plurality of subunits κ, where κdenotes the κth subunit in a chain of m subunits making up the daughterstrand, where K=1, 2, 3 to m, where m>10, generally m>50, and typicallym>500 or >5,000. Each subunit is formed of a tether (620) in itsexpanded configuration and is stretched to its length between the χlinkages of adjacent subunits. The primary backbone has been removedcompletely. Each subunit, a subunit motif α, contains species-specificgenetic information established by template-directed assembly of theXpandomer intermediate (daughter strand).

FIG. 62D shows the substrate construct of FIG. 62A as a molecular model,where the monomer substrate member, represented with a nucleobaseresidue (626), linked by a selectively cleavable linkage (625) to thetether first end moiety. Also disposed on the first end moiety is alinker group (629), shown as in FIG. 62A. A second linker group (628),shown as δ in FIG. 62A, is disposed on the second end moiety at thedistal end of the tether. A selectively cleavable intra-tether linkage(627), represented by the adjoining triangles, is shown that constrainsthe tether by linking the first and second end moiety. The linker groupsε (629) and δ (628) are positioned to not interact and to preferablyalign near the R¹ and R² sides of the substrate, respectively. Thetether loop shown here has three reporters (900,901,902), which can alsobe motif species specific.

FIG. 62E shows the substrate construct after incorporation into theproduct Xpandomer. The subunits are cleaved and expanded and are linkedby χ bonds (910), depicted as an open oval, formed by linking the linkergroups δ and ε referred to in FIG. 62A. A subunit is indicated by dottedlines vertically bracketing the repeating subunit, as represented bybrackets in the accompanying FIG. 62C.

In the Xpandomer product of FIG. 62E, the primary backbone has beendissociated or fragmented and is separated from the Xpandomer. Throughthe cleavage process, the constrained Xpandomer is released to becomethe Xpandomer product. The tether members that were formerly inconstrained configuration are now in expanded configuration, therebyfunctioning to linearly stretch out the sequence information of thetemplate target. Expanding the tethers lowers the linear density of thesequence information along the Xpandomer and provides a platform forincreasing the size and abundance of reporters which in turn improvessignal to noise for detection and decoding of the template sequence.

While the tether is depicted as a reporter construct with three reportergroups, various reporter configurations can be arrayed on the tether,and can comprise single reporters that identify monomer or the tethercan be a naked polymer. In some cases, one or more reporter precursorsare arrayed on the tether, and reporters are affinity bound orcovalently bound following assembly of the Xpandomer product.

Class VIII Monomeric Constructs

Class VIII molecules are analogs of Class VI described previously. Theprimary difference is that the linker group, ε, is connected directly tothe substrate instead of to the tether. In FIG. 63, we describe ClassVIII monomeric substrate constructs (a type of RT-NTP), in more detail.FIGS. 63A through 63C are read from left to right, showing first themonomeric substrate construct (Xpandomer precursor having a singlenucleobase residue), then the intermediate duplex daughter strand in thecenter, and on the right the Xpandomer product prepared for sequencing.

As shown in FIG. 63A, the monomeric substrate constructs of Class VIIIhave a tether, T (630), attached by a linkage (631) of a first endmoiety to a substrate nucleobase residue, N. At the distal end of thetether (632), a second end moiety with a second linker group, δ, ispositioned preferentially proximate to R². The second end moiety of thetether is secured to the first end moiety in proximity to the nucleobaseby an intra-tether selectively cleavable crosslink (or by otherconstraint). The intra-tether cleavable crosslink (633) is denoted hereby dotted line, which can indicate, for example, a disulfide bond or aphotocleavable linker. This constraint prevents the tether fromelongating or expanding and is said to be in its “constrainedconfiguration”. A linker group, ε (635), is attached to the monomersubstrate preferentially proximate to R¹. Under template-directedassembly, substrates form a duplex with the target template such thatthe substrates are abutted. Under controlled conditions, linker groups δand ε of the abutting substrates are collocated and link to form a bondbetween the adjacent substrate constructs. Linker groups δ and ε of amonomeric substrate construct do not form an intra-substrate bond due topositioning constraints. Suitable linkage and protection/deprotectionchemistries for δ, ε, and χ are detailed in the general monomericconstruct description.

-   -   R¹ and R² are end groups configured as appropriate for the        synthesis protocol in which the substrate construct is used. For        example, R¹=5′-phosphate and R²=3′-OH, would find use in a        ligation protocol, and R¹=5′-triphosphate and R²=3′-OH for a        polymerase protocol. Optionally, R² can be configured with a        reversible blocking group for cyclical single-substrate        addition. Alternatively, R¹ and R² can be configured with linker        end groups for chemical coupling or with no linker groups for a        hybridization only protocol. R¹ and R² can be of the general        type XR, wherein X is a linking group and R is a functional        group.

During assembly, the monomeric substrate construct is first polymerizedon the extendable terminus of the nascent daughter strand by a processof template-directed polymerization using a single-stranded template asa guide. Generally, this process is initiated from a primer and proceedsin the 5′ to 3′ direction. Generally, a DNA polymerase or otherpolymerase is used to form the daughter strand, and conditions areselected so that a complementary copy of the template strand isobtained. Subsequently, linker group δ, which is now collocated with thelinker group ε of the adjacent subunit tether, is caused to crosslinkand forms a χ-bond, which is an inter-subunit bond. The χ-bonds providea second linkage between subunits (polymerized inter-substrate linkagesare the first) and form an intermediate termed the “duplex daughterstrand”, as shown in FIG. 63B.

The duplex daughter strand (FIG. 63B) is a hetero-copolymer withsubunits shown in brackets. The primary backbone (˜N˜)κ, template strand(—N′-)κ, and tether (T) are shown as a duplexed daughter strand, where Kdenotes a plurality of repeating subunits. Each subunit of the daughterstrand is a repeating “motif” and the motifs have species-specificvariability, indicated here by the α superscript. The daughter strand isformed from monomeric substrate construct species selected by atemplate-directed process from a library of motif species, the monomersubstrate of each substrate construct species binding to a correspondingcomplementary nucleotide on the target template strand. In this way, thesequence of nucleobase residues (i.e., primary backbone) of the daughterstrand is a contiguous, complementary copy of the target templatestrand.

Each tilde (˜) denotes a selectively cleavable bond shown here as theinter-substrate bonds. These are necessarily selectively cleavable torelease and expand the tethers (and the Xpandomer) without degrading theXpandomer itself.

The daughter strand is composed of an Xpandomer precursor called the“constrained Xpandomer” which is further composed of tethers in the“constrained configuration”. When the tethers convert to their “expandedconfiguration”, the constrained Xpandomer converts to the Xpandomerproduct. The tethers are constrained by the inter-subunit χ linkages,the substrate attachments and, optionally, the intra-tether linkages ifstill present. The χ linkage attaches the substrate of a first subunitto the second end moiety tether at the abutting end of a second subunitand is formed by linking the collocated linker groups, ε of the firstsubunit, and δ of the second subunit.

The daughter strand can be seen to have two backbones, a “primarybackbone”, and the backbone of the “constrained Xpandomer”. The primarybackbone is composed of the contiguously abutted and polymerizedmonomeric substrates. The “constrained Xpandomer backbone” bypasses theselectively cleavable linkage between the monomer substrates and isformed by χ bond linked backbone moieties, each backbone moiety being atether linked to a substrate which then links to the next backbonemoiety tether with a χ bond. It can be seen that the constrainedXpandomer backbone bridges over the selectively cleavable bonds of theprimary backbone, and will remain covalently intact when theseselectively cleavable bonds are cleaved and the primary backbone isfragmented.

The tether χ bond (crosslinking of linker groups δ and ε) is generallypreceded by enzymatic coupling of the monomer substrates to form theprimary backbone, with, for example, phosphodiester bonds betweenadjacent bases. In the structure shown here, the daughter strand primarybackbone has been formed, and the inter-substrate linkages are depictedby a tilde (˜) to indicate that they are selectively cleavable. Afterdissociating or degrading the target template strand, cleaving theselectively cleavable bonds (which include the intra-tether bonds), theconstrained Xpandomer is released and becomes the Xpandomer product.Methods for dissociation of the template strand include heatdenaturation, or selective digestion with a nuclease, or chemicaldegradation. A method for selectable cleavage uses nuclease digestionwhere for example, phosphodiester bonds of the primary backbone aredigested by a nuclease and tether-to-tether bonds are nucleaseresistant.

FIG. 63C is a representation of the Class VIII Xpandomer product afterdissociation of the template strand and after cleavage of theselectively cleavable bonds (including those in the primary backboneand, if not already cleaved, the intra-tether links). The Xpandomerproduct strand contains a plurality of subunits κ, where κ denotes theκ^(th) subunit in a chain of m subunits making up the daughter strand,where κ=1, 2, 3 to m, where m>10, generally m>50, and typically m>500or >5,000. Each subunit is formed of a tether in its expandedconfiguration and is stretched to its length between the χ linkages ofadjacent subunits. The pendant substrate is attached to the tether ineach subunit. Each subunit, a subunit motif α, contains species-specificgenetic information established by template-directed assembly of theXpandomer intermediate (daughter strand).

FIG. 63D shows the substrate construct of FIG. 63A as a molecular model,where the monomer substrate member, represented with a nucleobaseresidue (634), is joined to the tether by a linkage (631) of the tetherfirst end moiety. A second linker group (639), shown as δ in FIG. 63A,is disposed on the second end moiety at the distal end of the tether. Aselectively cleavable intra-tether linkage (633), depicted by theadjoining triangles, is shown that constrains the tether by linking thefirst and second end moiety. Also attached to the substrate is a linkergroup (638), shown ε as in FIG. 63A. The linker groups ε (638) and δ(639) are positioned to not interact and to preferably align near the R¹and R² sides of the substrate respectively. The tether loop shown herehas three reporters (900,901,902), which can also be motif speciesspecific. The cleavable intra-tether crosslink (633) is shown in FIG.63D and FIG. 63E to be positioned closer to the substrate than the δ(639). In FIG. 60D and FIG. 60E the positions are switched. Thispositioning can be in either place and in one embodiment both linkerfunctions may be on a single multifunctional group.

FIG. 63E shows the substrate construct after incorporation into theproduct Xpandomer. The subunits are cleaved and expanded and are linkedby χ bonds (970,971), depicted as an open oval, formed by linking thelinker groups δ and ε referred to in FIG. 63A. A subunit is indicated bydotted lines vertically bracketing the repeating subunit, as representedby brackets in the accompanying FIG. 63C.

In the Xpandomer product of FIG. 63E, the primary backbone has beenfragmented and is not covalently contiguous because any direct bondbetween the substrates of adjacent subunits has been cleaved. Throughthe cleavage process, the constrained Xpandomer is released to becomethe Xpandomer product. The tether members that were formerly inconstrained configuration are now in expanded configuration, therebyfunctioning to linearly stretch out the sequence information of thetemplate target. Expanding the tethers lowers the linear density of thesequence information along the Xpandomer and provides a platform forincreasing the size and abundance of reporters which in turn improvessignal to noise for detection and decoding of the template sequence.

While the tether (630) is depicted as a reporter construct with threereporter groups, various reporter configurations can be arrayed on thetether, and can comprise single reporters that identify monomer or thetether can be a naked polymer. In some cases, one or more reporterprecursors are arrayed on the tether, and reporters are affinity boundor covalently bound following assembly of the Xpandomer product.

Class IX Monomeric Constructs

A class IX substrate construct is distinguished from the other RT-NTPsbecause it has two tether attachment points to which a free tether isattached after the primary backbone has been assembled. FIG. 64describes Class IX monomeric substrate constructs (a type of RT-NTP), inmore detail. FIGS. 64A through 64C are read from left to right, showingfirst the monomeric substrate construct (Xpandomer precursor having asingle nucleobase residue), then the intermediate duplex daughter strandin the center, and on the right the Xpandomer product prepared forsequencing.

As shown in FIG. 64A, the monomeric substrate construct of Class IX hasa substrate nucleobase residue, N, with two tether attachment sites, thelinker groups δ₁ and δ₂. Also shown is a free tether, T (640), withlinker groups ε₁ and ε₂ of a first and second tether end moiety. Thefirst and second end moieties of the tether constrain the free tether byan intra-tether selectively cleavable crosslink (647) and serves tocollocate linker groups ε₁ and ε₂. The cleavable crosslink is denotedhere by dotted line, can indicate, for example, a disulfide bond or aphotocleavable linker. This constraint prevents the tether fromelongating or expanding and is said to be in its “constrainedconfiguration”. Linker groups, δ₁ and δ₂, are attached to the monomersubstrate oriented proximate to R¹ and R², respectively. Undertemplate-directed synthesis, substrates form a duplex with the targettemplate and the linker group of one substrate construct, for exampleδ₁, and the linker group of the abutted substrate construct, for exampleδ₂, are collocated. Under controlled conditions, these collocatedlinkers contact the collocated ε linkers on the end of the free tether.Two selective linkages reactions occur, ε₁ and δ₁ to form χ¹ and ε₂ andδ₂ to form χ²; the adjacent substrates are now bridged by theconstrained tether. Suitable linkage and protection/deprotectionchemistries for δ, ε, and χ are detailed in the general monomericconstruct description.

R¹ and R² are end groups configured as appropriate for the synthesisprotocol in which the substrate construct is used. For example,R¹=5′-phosphate and R²=3′-OH, would find use in a ligation protocol, andR¹=5′-triphosphate and R²=3′-OH for a polymerase protocol. Optionally,R² can be configured with a reversible blocking group for cyclicalsingle-substrate addition. Alternatively, R¹ and R² can be configuredwith linker end groups for chemical coupling or with no linker groupsfor a hybridization only protocol. R¹ and R² can be of the general typeXR, wherein X is a linking group and R is a functional group.

During assembly, the monomeric substrate construct (without tether) ispolymerized on the extendable terminus of the nascent daughter strand bya process of template-directed polymerization using a single-strandedtemplate as a guide. Generally, this process is initiated from a primerand proceeds in the 5′ to 3′ direction. Generally, a DNA polymerase orother polymerase is used to form the daughter strand, and conditions areselected so that a complementary copy of the template strand isobtained. Linker group δ₁ is now collocated with the linker group δ₂ ofthe adjacent monomer substrate. After polymerization of the primarybackbone, free tethers are crosslinked to form χ¹-bonds and χ²-bondsbetween the two tether ends and two the adjacent substrates.

In one embodiment, the free tethers have no sequence information and aretermed “naked”. In this case, a single linker chemistry can be used solinker groups δ₁ and δ₂ are the same and linker groups ε₁ and ε₂ are thesame with one χ-bond type.

In the embodiment where the free tether comprises base type information(as in reporters) there are species-specific free tethers. Generally,there are four base types that would require four free tether types withthe corresponding base information. Several methods can be used to linkthe correct free tether species to its correct base. In one method, fourheterospecific linkage chemistries are used to further differentiate thelinker pair δ₁ and ε₁, now expressed as δ_(1α) and ε_(1α) where a is oneof four types and form bond types χ^(1α). δ_(1α) and ε_(1α) of one atype will bond only with each other. In this method the linker pair δ₂and ε₂ are caused to bond only after the δ_(1α) and ε_(1α) has beenformed. In a second method, different selectively deprotectableprotection groups, where each protection group is associated with onelinker type, are used to selectively block linker groups. In a firstcycle, there is no protection on one base type and its associated freetether type is bonded ε₁ to δ₁ to form a χ¹-bond. In a second cycle, onetype of protection group is removed from one base type and itsassociated free tether type is bonded ε₁ to δ₁ to form a χ1-bond. Thislatter cycle is repeated for the next two base types and after it iscompleted, the linker pair δ₂ and ε₂ are caused to bond. Between eachstep wash steps are included to reduce bonding errors. Without loss ofgenerality, the remaining description will be in terms of χ¹-bonds andχ²-bonds. Note that after the tether is attached and the χ-bonds areformed, the intra-tether bond may be broken.

As shown in FIG. 64B, the χ¹-bonds provide a linkage between subunits(in addition to the polymerized inter-substrate linkages) and form anintermediate termed the “duplex daughter strand”. The primary backbone(˜N˜)κ, template strand (—N′-)κ, and Tether (T) are shown as a duplexeddaughter strand, where κ denotes a plurality of repeating subunits. Eachsubunit of the daughter strand is a repeating “motif” and the motifshave species-specific variability, indicated here by the α superscript.The daughter strand is formed from monomeric substrate construct speciesselected by a template-directed process from a library of motif species,the monomer substrate of each substrate construct species binding to acorresponding complementary nucleotide on the target template strand. Inthis way, the sequence of nucleobase residues (i.e., primary backbone)of the daughter strand is a contiguous, complementary copy of the targettemplate strand.

Each tilde (˜) denotes a selectively cleavable bond shown here as theinter-substrate bonds. These are necessarily selectively cleavable torelease and expand the tethers (and the Xpandomer) without degrading theXpandomer itself.

The daughter strand is composed of an Xpandomer precursor called the“constrained Xpandomer” which is further composed of tethers in the“constrained configuration”. When the tethers convert to their “expandedconfiguration”, the constrained Xpandomer converts to the Xpandomerproduct. The tethers are constrained by the χ linkages to adjacentsubstrates and optionally, the intra-tether linkages (if still present).

The daughter strand can be seen to have two backbones, a “primarybackbone”, and the backbone of the “constrained Xpandomer”. The primarybackbone is composed of the contiguously abutted and polymerizedmonomeric substrates. The “constrained Xpandomer backbone” bypasses theselectively cleavable linkage between monomer substrates and is formedby χ¹-bond linked backbone moieties, each backbone moiety being a tetherlinked to a substrate (by a χ²-bond) which then links to the nextbackbone moiety tether with a χ¹-bond. It can be seen that theconstrained Xpandomer backbone bridges over the selectively cleavablebonds of the primary backbone, and will remain covalently intact whenthese selectively cleavable bonds are cleaved and the primary backboneis fragmented.

The tether χ bond (crosslinking of linker groups δ and ε) is generallypreceded by enzymatic coupling of the monomer substrates to form theprimary backbone, with, for example, phosphodiester bonds betweenadjacent bases. In the structure shown here, the daughter strand primarybackbone has been formed, and the inter-substrate linkages are depictedby a tilde (˜) to indicate that they are selectively cleavable. Afterdissociating or degrading the target template strand, cleaving theselectively cleavable bonds (which include the intra-tether bonds), theconstrained Xpandomer is released and becomes the Xpandomer product.Methods for dissociation of the template strand include heatdenaturation, or selective digestion with a nuclease, or chemicaldegradation. A method for selectable cleavage uses nuclease digestionwhere for example, phosphodiester bonds of the primary backbone aredigested by a nuclease and tether-to-tether bonds are nucleaseresistant.

FIG. 64C is a representation of the Class IX Xpandomer product afterdissociation of the template strand and after cleavage of theselectively cleavable bonds (including those in the primary backboneand, if not already cleaved, the intra-tether links). The Xpandomerproduct strand contains a plurality of subunits κ, where κ denotes theκ^(th) subunit in a chain of m subunits making up the daughter strand,where κ=1, 2, 3 to m, where m>10, generally m>50, and typically m>500or >5,000. Each subunit is formed of a tether (640) in its expandedconfiguration linked to a monomer substrate and to the χ¹ linkage withthe next adjacent subunit. Each subunit, a subunit motif α, containsspecies-specific genetic information established by template-directedassembly of the Xpandomer intermediate (daughter strand).

FIG. 64D shows the substrate construct of FIG. 64A as a molecular model,where the monomer substrate member, represented with a nucleobaseresidue (641), has two linkages (642,643), shown in FIG. 64 a as δ₁ andδ₂, that will be attachment points for the free tether. The free tetheris shown with two linker groups linker group (644,645), shown as ε₁ andε₂ in FIG. 64A, on the first and second end moieties of the tether. Aselectively cleavable intra-tether linkage (647) is shown thatconstrains the tether by linking the first and second end moieties. Thelinker groups are positioned to foster crosslinking of the tether endsbetween the monomer substrates and prevent crosslinking across a monomersubstrate.

The tether loop shown here has three reporters (900,901,902), which canalso be motif species specific but requires a method to be correctlylinked to the correct bases in the primary backbone.

FIG. 64E shows the substrate construct after incorporation into theproduct Xpandomer. The subunits are cleaved and expanded and are linkedby inter-tether χ¹ bonds (930,931), and inter-tether χ² bonds (932,933).Each subunit is a tether linked to a monomer substrate and connected onto the next χ¹ bond. A subunit is indicated by dotted lines verticallybracketing the repeating subunit, as represented by brackets in theaccompanying FIG. 64C.

In the Xpandomer product of FIG. 64E, the primary backbone has beenfragmented and is not covalently contiguous because any direct bondbetween the substrates of adjacent subunits has been cleaved. Throughthe cleavage process, the constrained Xpandomer is released to becomethe Xpandomer product. The tether members that were formerly inconstrained configuration are now in expanded configuration, therebyfunctioning to linearly stretch out the sequence information of thetemplate target. Expanding the tethers lowers the linear density of thesequence information along the Xpandomer and provides a platform forincreasing the size and abundance of reporters which in turn improvessignal to noise for detection and decoding of the template sequence.

While the tether is depicted as a reporter construct with three reportergroups, various reporter configurations can be arrayed on the tether,and can comprise single reporters that identify monomer or the tethercan be a naked polymer. In some cases, one or more reporter precursorsare arrayed on the tether, and reporters are affinity bound orcovalently bound following assembly of the Xpandomer product.

FIG. 65 demonstrates a method synthesizing Xpandomers using RT-NTPsubstrate constructs of Class IX. A target template (650) is firstselected and annealed to an immobilized primer. In Step I, the primer isextended by template-directed synthesis of a daughter strand. Thisprocess continues in Step II, and in FIG. 65 a, a magnified view (dottedarrow) is shown of the daughter strand. Illustrated are the templatestrand, primer, and polymerized Class IX nucleobase substrate constructs(without tethers), each substrate construct with chemicalfunctionalities, depicted as the lock and key symbols, for chemicaladdition of tether reagents. The functionalities are selected so thateach monomer has a base-specific tether attachment site and a universalattachment site. In step III, hairpin tethers with four species specificlinkers are introduced. The tethers link to the primary backbone in abase specific manner according to the base specific linkers as shown inFIG. 65 b, a magnified view (dotted arrow). Here, white and blackcircles indicate the universal chemical attachment chemistry and diamondand fork shapes indicate the base-specific attachment chemistries. InStep IV and FIG. 65 c, a magnified view (dotted arrow), after all basespecific linking is complete, the universal linkers are caused to formbonds. Note that the universal linkers on both the tethers and theprimary backbone are in close proximity to their base specific linkercounterpart to avoid linking errors. The chemical attachment of thetether reagents is shown as having been completed, and a constrainedXpandomer has been formed on the template. The Xpandomer is thenreleased (not shown) by dissociating from the template, and cleaving theselectively cleavable bonds (of primary backbone and intra-tetherbonds).

Class X Monomeric Constructs

Class X substrate constructs, also called XNTPs, differ from the RT-NTPsin that the tether is contained within each monomer substrate to form anintra-substrate tether, each XNTP substrate having a selectivelycleavable bond within the substrate which, once cleaved, enables theexpansion of the constrained tether. In FIG. 66, we describe Class Xmonomeric substrate constructs in more detail. FIGS. 66A through 66C areread from left to right, showing first the monomeric substrate construct(Xpandomer precursor having a single nucleobase residue), then theintermediate duplex daughter strand in the center, and on the right theXpandomer product prepared for sequencing.

As shown in FIG. 66A, the monomeric substrate construct of Class X has asubstrate nucleobase residue, N, that has two moieties (662,663)separated by a selectively cleavable bond (665), each moiety attachingto one end of a tether (660). The tether ends can attach to the linkergroup modifications on the heterocycle, the ribose group, or thephosphate backbone. The monomer substrate also has an intra-substratecleavage site positioned within the phosphororibosyl backbone such thatcleavage will provide expansion of the constrained tether. For example,to synthesize a Class X ATP monomer, the amino linker on8-[(6-Amino)hexyl]-amino-ATP or N6-(6-Amino)hexyl-ATP can be used as afirst tether attachment point, and, a mixed backbone linker, such as thenon-bridging modification (N-1-aminoalkyl) phosphoramidate or(2-aminoethyl) phosphonate, can be used as a second tether attachmentpoint. Further, a bridging backbone modification such as aphosphoramidate (3′ O—P—N 5′) or a phosphorothiolate (3′ O—P—S 5′), forexample, can be used for selective chemical cleavage of the primarybackbone.

-   -   R¹ and R² are end groups configured as appropriate for the        synthesis protocol in which the substrate construct is used. For        example, R¹=5′-triphosphate and R²=3′-OH for a polymerase        protocol. The R¹ 5′ triphosphate may include mixed backbone        modifications, such as an aminoethyl phosphonate or 3′-O—P—S-5′        phosphorothiolate, to enable tether linkage and backbone        cleavage, respectively. Optionally, R² can be configured with a        reversible blocking group for cyclical single-substrate        addition. Alternatively, R¹ and R² can be configured with linker        end groups for chemical coupling. R¹ and R² can be of the        general type XR, wherein X is a linking group and R is a        functional group.

During assembly, the monomeric substrate construct is polymerized on theextendable terminus of the nascent daughter strand by a process oftemplate-directed polymerization using a single-stranded template as aguide. Generally, this process is initiated from a primer and proceedsin the 5′ to 3′ direction. Generally, a DNA polymerase or otherpolymerase is used to form the daughter strand, and conditions areselected so that a complementary copy of the template strand isobtained.

As shown in FIG. 66B, the nucleobase residues are polymerized onesubunit to the next and form an intermediate termed the “duplex daughterstrand”. The primary backbone (—N—)κ, template strand (—N′-)κ, andTether (T) are shown as a duplexed daughter strand, where K denotes aplurality of repeating subunits. Each subunit of the daughter strand isa repeating “motif” and the motifs have species-specific variability,indicated here by the a superscript. The daughter strand is formed frommonomeric substrate construct species selected by a template-directedprocess from a library of motif species, the monomer substrate of eachsubstrate construct species binding to a corresponding complementarynucleotide on the target template strand. In this way, the sequence ofnucleobase residues (i.e., primary backbone) of the daughter strand is acontiguous, complementary copy of the target template strand.

The (“V”) shown in FIG. 66B above the nucleobase residue denotes aselectively cleavable bond that divides the substrate into the first andsecond moieties. Upon cleaving, the first moiety (669) of one subunitwill remain linked to the second moiety (668) of the adjacent subunitand within a subunit, each moiety will be bridged, one to the other bythe tether. These are necessarily selectively cleavable to release andexpand the tethers (and the Xpandomer) without degrading the Xpandomeritself.

The daughter strand has two backbones, a “primary backbone”, and thebackbone of the “constrained Xpandomer”. The primary backbone iscomposed of the contiguously abutted and polymerized monomericsubstrates. The “constrained Xpandomer backbone” bypasses theselectively cleavable linkage within the monomer substrate and is formedby the inter-substrate bonds linking the backbone moieties, eachbackbone moiety being a tether linked to two moieties of the stillintact nucleobase residue. The constrained Xpandomer backbone bridgesover the selectively cleavable bond within each monomer, and will remaincovalently intact when these selectively cleavable bonds are cleaved andmonomers are fragmented into portions n¹ and n² shown in FIG. 66C.

Cleaving is preceded by enzymatic coupling of the monomer substrates toform the primary backbone, with, for example, phosphodiester or mixedbackbone bonds between adjacent bases. In the structure shown here, thedaughter strand primary backbone has been formed. After dissociating ordegrading the target template strand and cleaving the selectivelycleavable bonds, the constrained Xpandomer is released and becomes theXpandomer product. Methods for dissociation of the template strandinclude for example heat denaturation.

FIG. 66C is a representation of the Class X Xpandomer product afterdissociation of the template strand and after cleavage of theselectively cleavable bonds. The Xpandomer product strand contains aplurality of subunits κ, where κ denotes the κ^(th) subunit in a chainof m subunits making up the daughter strand, where κ=1, 2, 3 to m, wherem>10, generally m>50, and typically m>500 or >5,000. Each subunit isformed of a tether in its expanded configuration linked to portions n¹and n² of a monomer substrate, and each subunit is linked to the next bythe monomer polymerization bonds. Each subunit, a subunit motif a,contains species-specific genetic information established bytemplate-directed assembly of the Xpandomer intermediate (daughterstrand).

FIG. 66D shows the substrate construct as a molecular model, where thenucleobase member (664) is joined to a first and second moiety of thenucleobase, each moiety with an attachment site to the tether (662,663).The tether (660) comprises reporter groups (900,901,902). A selectivelycleavable bond separating the two moieties of the nucleobase is denotedby a “V” (665).

FIG. 66E shows the Xpandomer product. The subunits comprise the expandedtether (660) attached to nucleobase portions (669,668), shown as n¹ andn² in FIG. 66C, each subunit joined by inter-nucleobase bonds. Throughthe cleavage process, the constrained Xpandomer is released to becomethe Xpandomer product. The tether members that were formerly inconstrained configuration, are now in expanded configuration, therebyfunctioning to linearly stretch out the sequence information of thetemplate target. Expanding the tethers lowers the linear density of thesequence information along the Xpandomer and provides a platform forincreasing the size and abundance of reporters which in turn improvessignal to noise for detection and decoding of the template sequence.

While the tether is depicted as a reporter construct with three reportergroups, various reporter configurations can be arrayed on the tether,and can comprise single reporters that identify monomer or the tethercan be a naked polymer. In some cases, one or more reporter precursorsare arrayed on the tether, and reporters are affinity bound orcovalently bound following assembly of the Xpandomer product.

FIG. 67 shows a method for assembling an Xpandomer with the XNTPsubstrate constructs of Class X. In the first view, a hairpin primer isused to prime a template and the template is contacted with a polymeraseand Class X monomer substrates. Polymerization is shown in Step I toextend the nascent daughter strand processively by template-directedaddition of monomer substrates. The magnified view (FIG. 67 a)illustrates this in more detail. The daughter strand complementary tothe template strand is shown to be composed of modified nucleobasesubstrates with internal cleavage site (“V”). In Step II, the process offorming the Xpandomer intermediate is completed, and in Step III, aprocess of cleavage, dissociation of the intermediate and expansion ofthe daughter strand is shown as underway. In the magnified view (FIG. 67b), internal cleavage of the nucleobase substrates is shown to relievethe constraint on the tethers, which expand, elongating the backbone ofthe Xpandomer.

EXAMPLE 1 Synthesis of a Chimeric 2mer Xprobe “CA” with SelectivelyCleavable Ribosyl-5′-3′ Internucleotide Bond

Oligomeric Substrate constructs are composed of probe members and tethermembers and have a general “probe-loop” construction. Probe membersynthesis is accomplished using well established methods of solid phaseoligomer synthesis. In these methods, addition of nucleobases to anascent probe chain on a resin is accomplished with phosphoramiditechemistry (U.S. Pat. Nos. 4,415,732 and 4,458,066) for example, andmilligrams or grams of synthetic oligomer can be economicallysynthesized using readily available automated synthesizers. Typicalsolid-phase oligonucleotide synthesis involves reiteratively performingfour steps: deprotection, coupling, capping, and oxidation. However, atleast one bond in the probe of a class I Xprobe substrate construct is aselectively cleavable bond, and at least two probe moieties are modifiedfor acceptance of a tether member. The selectively cleavable bond islocated between the probe moieties selected for tether attachment (i.e.,“between” should not be limited to mean “between adjacent nucleobasemembers” because the first and second points of tether attachment needonly be positioned anywhere on a first and second moiety of the probe,respectively, the moieties being joined by the selectively cleavablebond). In this example, a ribosyl 5′-3′ internucleotide bond, which isselectively cleavable by Ribonuclease H, is the selectively cleavablebond and the two points of tether attachment are the first and secondnucleobase residues of a 2mer probe.

Synthesis of linker modified Xprobes is achieved using commerciallyavailable phosphoramidites, for example, from Glen Research (Sterling,Va., USA), BioGenex (San Ramon, Calif., USA), Dalton ChemicalLaboratories (Toronto, Canada), Thermo Scientific (USA), LinkTechnologies (UK), and others, or can be custom synthesized. Wellestablished synthesis methods can be used to prepare the described probewherein the 3′ nucleobase, which for this example is an amino modifierC6 deoxyadenosine, is first attached to a universal support using5′-Dimethoxytrityl-N-6-benzoyl-N-8-[6-(trifluoroacetylamino)-hex-1-yl]-8-amino-2′-deoxyAdenosine-3′-[(2-cyanoethyl)-(N,N-diisopropyl)]-phosphoramiditefollowed by the addition of an amino modifier C6 cytidine using5′-Dimethoxytrityl-N-dimethylformamidine-5-[N(trifluoroacetylaminohexyl)-3-acrylimido]-Cytidine,3′-[(2-cyanoethyl)-(N,N-diisopropyl)]-phosphoramidite, wherein the 5′cytidine is a ribonucleotide. The addition of a chemical phosphorylationreagent followed by standard cleavage, deprotection, and purificationmethods completes the synthesis. The dinucleotide product is a 5′phosphate (aminoC6-Cytosine) (aminoC6-deoxyAdenosine) 3′ with acentrally cleavable ribosyl-5′, 3′ bond and amino linkers on each base.

The Xprobe tether for this example is constructed frombis-N-succinimidyl-[pentaethylene glycol]ester (Pierce, Rockford Ill.;Product No 21581). The linker amines of the modified pCA oligomer arecrosslinked with bis(NHS) PEG5 according to the manufacturer'sinstructions. A product of the expected molecular weight for thecircularized PEG-probe construct is obtained.

EXAMPLE 2 Synthesis of a 4mer Xprobe “TATA” with Selectively CleavablePhosphorothiolate Bond

Xprobe 4mers can be synthesized with a phosphorothiolate linkage as theselectively cleavable bond. For the following example, synthesis of a 5′phosphate (dT) (aminoC6-dA) (dT) (aminoC6-dA) 3′ tetranucleotide isdescribed.

A 5′ mercapto-deoxyThymidine is first prepared as described by Mag etal. (“Synthesis and selective cleavage of an oligodeoxynucleotidecontaining a bridged internucleotide 5′-phosphorothioate linkage”, NuclAcids Res 19:1437-41, 1991). Thymidine is reacted with two equivalentsof p-toluenesulfonyl chloride in pyridine at room temperature, and theresulting 5′-tosylate is isolated by crystallization from ethanol. Thetosylate is converted to a 5′-(S-trityl)-mercapto-5′-deoxy-thymidinewith five equivalents of sodium tritylthiolate (prepared in situ). The5′-(S-trityl)-mercapto-thymidine nucleotide is purified and reacted with2-cyanoethoxy-bis-(N,N-diisopropylamino-phosphane) in the presence oftetrazole to make the 3′-O-phosphoramidite building block.

To begin automated synthesis, amino modifier C6 deoxyadenosine is firstattached to a universal support using5′-Dimethoxytrityl-N-6-benzoyl-N-8-[6-(trifluoroacetylamino)-hex-1-yl]-8-amino-2′-deoxyAdenosine-3′-[(2-cyanoethyl)-(N,N-diisopropyl)]-phosphoramiditefollowed by the addition of mercaptothymidine phosphoramidite preparedabove. Prior to adding the next amino C6 dA phosphoramidite, theS-trityl group is first deprotected with 50 mM aqueous silver nitrateand the resin is washed with water. The resin is typically then treatedwith a reducing agent such as DTT to eliminate incidental disulfidesformed during the cleavage. The column is then again washed with waterand with acetonitrile, and the free thiol is reacted under standardconditions with the amino C6 deoxyAdenosine phosphoramidite in thepresence of tetrazole, thereby forming “ATA” with a bridging S3′→P5′phosphorothiolate bond between the terminal deoxyAdenosine and the3-mercapto-Thymidine. In the next cycle, standard deoxythymidinephosphoramidite is added. Finally, the addition of a chemicalphosphorylation reagent followed by standard cleavage, deprotection, andpurification methods completes the synthesis. The tetranucleotideproduct is a 5′ phosphate (dT) (aminoC6-dA) (dT) (aminoC6-dA) 3′.

The phosphorothiolate bond is selectively cleavable, for example, withAgCl, acid or with iodoethanol (Mag et al., “Synthesis and selectivecleavage of an oligodeoxynucleotide containing a bridged internucleotide5′-phosphorothioate linkage”, Nucleic Acids Research,19(7):1437-1441,1991). Because the selectively cleavable bond is betweenthe second and third nucleobases, the tether is designed to bridge thisbond, and may be attached to any two nucleobases (or any two primarybackbone attachment points) on either side of the selectively cleavablebond. Methods for zero linker and linker chemistries include, forexample, provision of linkers with primary amines used in the synthesisof the oligomer, as described in Example 1. The amine modified linkerson the “TATA” oligomer are typically protected during oligomer synthesisand are deprotected in the normal course of completing oligonucleotidesynthesis.

The Xprobe tether for this example is then constructed from bisepoxideactivated poly(ethylene glycol) diglycidyl ether (SigmaAldrich, St.Louis Mo., Product No 475696). The epoxide reaction of the amines withthe activated PEG end groups is conducted in dilute solution to minimizeany competing concatenation reaction. Similar reactivity is obtainedwith mixed anhydrides or even acid chlorides, and can employheterobifunctional linkage groups so as to orient the attachment of thetether. Tethered reaction products are separated by preparative HPLC andcharacterized by mass spectroscopy. A product of the approximatemolecular weight for the circularized 4mer PEG-probe construct (about2.5 Kd) is obtained. A distribution of PEG tethers with approximateM_(n)=500 is obtained with this method. This corresponds to a tether ofabout 40 Angstroms (at about 3.36 Å/PEG unit).

EXAMPLE 3 Synthesis of a 3mer Xprobe “CTA” with Selectively Cleavable5′-3′ Phosphodiester Bond

Xprobes can also be synthesized with a phosphodiester linkage as theselectively cleavable bond. For the following example, synthesis of a 5′phosphate (aminoC6-dC) (aminoC6-dT) (dA) 3′ trinucleotide with anon-bridging phosphorothioate modification is described. Phosphodiesterbonds are attacked by a variety of nucleases. A phosphorothioate bond,with non-bridging sulfur, is used as a nuclease-resistant bond in thisexample.

For automated synthesis in the 3′ to 5′ direction, a CPG immobilizeddeoxyAdenosine solid support is used(5′-Dimethoxytrityl-N-benzoyl-2′-deoxyAdenosine, 3′-succinoyl-long chainalkylamino-CPG 500). In the first cycle, amino modifier C6deoxyThymidine phosphoramidite(5′-Dimethoxytrityl-5-[N-(trifluoroacetylaminohexyl)-3-acrylimido]-2′-deoxyUridine,3′-[(2-cyanoethyl)-(N,N-diisopropyl)]-phosphoramidite) is coupled.Before capping, the immobilized dA is reacted with Sulfurizing Reagent(Glen Research, Sterling Va.; Cat No 40-4036), also known as BeaucageReagent, following the manufacturer's protocol. The reagent is generallyadded through a separate port in the synthesizer. Following thiolation,amino modified C6 deoxy Cytidine(5′-Dimethoxytrityl-N-dimethylformamidine-5-[N-(trifluoroacetylaminohexyl)-3-acrylimido]-2′-deoxyCytidine, 3′-[(2-cyanoethyl)-(N,N-diisopropyl)]-phosphoramidite) iscoupled. Before capping, the immobilized dC is reacted with SulfurizingReagent (Glen Research, Sterling Va.; Cat No 40-4036), also known asBeaucage Reagent, following the manufacturer's protocol. The resulting“CTA” 3mer has a phosphorothioate bond between the T and the A, and aphosphodiester bond between the C and the T. The addition of a chemicalphosphorylation reagent followed by standard cleavage, deprotection, andpurification methods completes the synthesis.

Resistance of phosphorothioate linkages to nuclease attack is wellcharacterized, for example by Matsukura et al (“Phosphorothioate analogsof oligodeoxynucleotides: inhibitors of replication and cytopathiceffects of human immunodeficiency virus”, PNAS 84:7706-10, 1987), byAgrawal et al (“Oliogodeoxynucleoside phosphoramidates andphosphorothioates as inhibitors of human immunodeficiency virus”, PNAS85:7079-83, 1988), and in U.S. Pat. No. 5,770,713. Both C and T arelinker or zero-linker modified, the derivatization serving forattachment of a tether member.

EXAMPLE 4 Synthesis of a 3mer Xprobe “ATA” with Selectively Cleavable 5′N—P—O 3′ Phosphoramidate Bond

Amino modifier C6 deoxyadenosine, is first attached to a universalsupport using5′-Dimethoxytrityl-N-6-benzoyl-N-8-[6-(trifluoroacetylamino)-hex-1-yl]-8-amino-2′-deoxyAdenosine-3′-[(2-cyanoethyl)-(N,N-diisopropyl)]-phosphoramiditefollowed in the next cycle by the addition of MMT-blocked 5′amino-dT(5′monomethooxytritylamino-2′-deoxyThymidine). After deblocking, the5′-amino end is reacted with amino modified C6 deoxyAdenosinephosphoramidite under standard conditions. The addition of a chemicalphosphorylation reagent followed by standard cleavage, deprotection, andpurification methods completes the synthesis. The trinucleotide 5′(aminoC6-dA) (O—P—N) (dT) (O—P—O) (aminoC6-dA) 3′ has a phosphoramidatebond between the 5′ aminoC6-dA and the penultimate dT.

This phosphoramidate bond is selectively cleavable under conditions inwhich phosphodiester bonds remain intact by treating the oligomer with80% acetic acid as described by Mag et al. (“Synthesis and selectivecleavage of oligodeoxynucleotides containing non-chiral internucleotidephosphoramidate linkages”, Nucl. Acids Res. 17: 5973-5988, 1989). Byattaching a tether to bridge the 5′N—P—O phosphoramidate bond, anXpandomer containing dimers of this type can be expanded by selectivecleavage of the phosphoramidate bonds of the primary backbone.

EXAMPLE 5 Synthesis of a 6mer Xprobe “CACCAC” with an InternalPhotocleavable Bond

After standard 3′ to 5′ synthesis with unmodified deoxycytosine anddeoxyAdenosine phosphoramidites, amino-modified C6 dC(5′-Dimethoxytrityl-N-dimethylformamidine-5-[N-(trifluoroacetylaminohexyl)-3-acrylimido]-2′-deoxyCytidine, 3′-[(2-cyanoethyl)-(N,N-diisopropyl)]-phosphoramidite; GlenResearch; Cat No 10-1019) is coupled, thereby forming a CAC trimer. Forthe next cycle, a photocleavable linker is coupled(3-(4,4′-Dimethoxytrityl)-1-(2-nitrophenyl)-propan-1-yl-[(2-cyanoethyl)-(N,N-diisopropyl)]-phosphoramidite;Glen Research; Cat No 10-4920). In the next cycle, a secondamino-modified dC is added followed by two final rounds of standardaddition of dA and dC phosphoramidites, respectively. The resultingproduct, “CAC-pc-CAC” contains amino-linkers at the third and fourthbase positions, and can be modified by addition of a tether that bridgesthe selectively cleavable bond formed by the photocleavable nitrobenzeneconstruct between the two amino-modified bases. Selective cleavage of aphotocleavable linker modified phosphodiester backbone is disclosed bySauer et al. (“MALDI mass spectrometry analysis of single nucleotidepolymorphisms by photocleavage and charge-tagging”, Nucleic AcidsResearch 31, 11 e63, 2003), Vallone et al. (“Genotyping SNPs using aUV-photocleavable oligonucleotide in MALDI-TOF MS”, Methods Mol. Bio.297:169-78, 2005), and Ordoukhanian et al. (“Design and synthesis of aversatile photocleavable DNA building block, application tophototriggered hybridization”, J. Am. Chem. Soc. 117, 9570-9571, 1995),for example.

EXAMPLE 6 Synthesis of an Xmer Substrate Construct

Xmer substrate constructs are closely related in design and compositionto Xprobes. An Xmer library is synthesized for example by5′-pyrophosphorylation of Xprobes. Established procedures forpyrophosphate treatment of 5′-monophosphates include, for example,Abramova et al. (“A facile and effective synthesis of dinucleotide5′-triphoshates”, Bioorganic Medicinal Chemistry 15: 6549-55, 2007). Inthis method the terminal monophosphate of the oligomer is activated asnecessary for subsequent reaction with pyrophosphate by first reactingthe terminal phosphate as a cetyltrimethylammonium salt with equimolarquantities of triphenylphosphine (Ph₃P) and 2,2′-dipyridyl disulfide(PyS)₂ in DMF/DMSO, using DMAP (4-dimethylaminopyridine) or 1-Melm(1-methylimidazole) as a nucleophilic catalyst. The product isprecipitated with LiClO₄ in acetone and purified by anion exchangechromatography.

A variety of other methods can be considered for robust synthesis of 5′triphosphate Xmers. As described by Burgess and Cook (Chem Rev100(6):2047-2060), these methods include, but are not limited to,reactions using nucleoside phosphoramidites, synthesis via nucleophilicattack of pyrophosphate on activated nucleoside monophosphates,synthesis via nucleophilic attack of phosphate on activated nucleosidepyrophosphate, synthesis via nucleophilic attack of diphosphate onactivated phosphate synthon, synthesis involving activated phosphites orphosphoramidites derived from nucleosides, synthesis involving directdisplacement of 5′-O-leaving groups by triphosphate nucleophiles, andbiocatalytic methods. One specific method that produced polymerasecompatible dinucleotide substrates uses N-methylimidazole to activatethe 5′ monophosphate group; subsequent reaction with pyrophosphate(tributylammonium salt) produces the triphosphate (Bogachev, 1996). Inanother procedure, trinucleotide phosphoramidates have been synthesizedby Kayushin (Kayushin A L et al. 1996. A convenient approach to thesynthesis of trinucleotide phosphoramidites. Nucl Acids Res 24:3748-55).

EXAMPLE 7 Xpandomer Synthesis with Polymerase Phosphoramidate Cleavage

Synthesis of an Xpandomer is performed using a substrate constructprepared with 5′-triphosphate and 3′-OH ends. U.S. Pat. No. 7,060,440 toKless describes use of polymerases to polymerize triphosphate oligomers,and the method is adapted here for synthesis of Xpandomers. Thesubstrate construct consists of 2mer probe member “pppCA” withinter-nucleotide 5′ N—P—O phosphoramidate selectively cleavable bond anda PEG tether loop construct. A template strand and companion primer issynthesized and is purified before use. The sequence:“TGTGTGTGTGTGTGTGTGTGATCTACCGTCCGTCCC” is used as a template. Thesequence “5′GGGACGGACGGTAGAT” is used as a primer. A 5′ terminal HEX(5′hexachloro-fluorescein) on the primer is used as a label. Annealingof the primer and template forms a template with duplex primer and free3′-OH and 5′ terminal single stranded overhang of twenty bases inlength. The substrate constructs and Sequenase™ brand recombinant T7 DNApolymerase (US Biochemicals Corp., Cleveland, Ohio) are then added andpolymerization is continued for 30 min under conditions adjusted foroptimum polymerization. A sample of the polymerization reaction is mixedwith gel loading buffer and is electrophoresed on a 20% TBE acrylamidegel (Invitrogen, USA) along with a no polymerase negative control and aMW marker to confirm Xmer polymerization.

The Xpandomer intermediate is treated with 80% acetic acid for 5 hrs atroom temperature according to the procedure of Mag et al. (“Synthesisand selective cleavage of oligodeoxyribonucleotides containingnon-chiral internucleotide phosphoramidate linkages”, Nucl. Acids Res.,17: 5973-88, 1989) to selectively cleave the phosphoramidate bonds,which is likewise confirmed by electrophoresis.

EXAMPLE 8 Xpandomer Synthesis with Polymerase Phosphorothioate Cleavage

Synthesis of an Xpandomer is performed using a substrate constructprepared with 5′-triphosphate and 3′-OH ends. U.S. Pat. No. 7,060,440 toKless describes use of polymerases to polymerize triphosphate oligomers.The substrate construct is a “pppCA”. The substrate construct isdesigned with a selectively cleavable inter-nucleotide phosphorothiolatebackbone linkage and a PEG 2000 tether loop construct. A template strandand companion primer is synthesized and is purified before use. Thesequence: “TGTGTGTGTGTGTGTGTGTGATCTACCGTCCGTCCC” is used as a targettemplate. The sequence “5′GGGACGGACGGTAGAT” is used as a primer. A 5′terminal HEX (5′hexachloro-fluorescein) on the primer is used as alabel. Annealing of the primer and template forms a template with duplexprimer and free 3′-OH and 5′ terminal single stranded overhang of twentybases in length. The substrate constructs and Therminator™ DNAPolymerase (New England Biolabs, USA) are then added with buffer andsalts optimized for polymerization. Polymerization continued for 60 minunder conditions adjusted for optimum polymerization. A sample of thepolymerization reaction is mixed with gel loading buffer and iselectrophoresed on a 20% TBE acrylamide gel (Invitrogen, USA) along witha no polymerase negative control and a MW marker to confirm Xmerpolymerization.

The phosphorothiolate bonds of the Xpandomer intermediate areselectively cleavable, for example, with AgCl, acid or with iodoethanol(Mag et al., “Synthesis and selective cleavage of anoligodeoxynucleotide containing a bridged internucleotide5′-phosphorothioate linkage”, Nucleic Acids Research,19(7):1437-1441,1991). Cleavage is confirmed by gel electrophoresis.

EXAMPLE 9 Chimeric Xpandomer Synthesis with Ligase

Synthesis of an Xpandomer is performed using a substrate constructprepared with 5′-monophosphate and 3′-OH ends. The substrate constructis a chimeric “5′ p dC rA˜dC dA 3′” 4mer where the 5′ penultimateadenosine is a ribonucleotide and the rest of the substrate isdeoxyribonucleotide. The substrate construct is designed with oneselectively cleavable inter-nucleotide ribosyl 5′-3′phosphodiester bonds(as shown by the “˜”) and a PEG 2000 tether loop construct, where thetether is attached at the terminal “C” and “A” of the 4mer. A templatestrand and companion primer is synthesized and is purified before use.The sequence: “TGTGTGTGTGTGTGTGTGTG ATCTACCGTCCGTCCC” is used as atarget template. The sequence “5′GGGACGGACGGTAGAT” is used as a primer.A 5′ terminal HEX (5′hexachloro-fluorescein) on the primer is used as alabel. Annealing of the primer and template forms a template with duplexprimer and free 3′-OH and 5′ terminal single stranded overhang of twentybases in length. The substrate constructs and T4 DNA ligase (PromegaCorp, Madison, Wis., USA; Cat No M1801) are then added with temperature,buffer and salts optimized for transient probe hybridization andligation. Ligation is continued for 6 hours. A sample of the ligationreaction is mixed with gel loading buffer and is electrophoresed on a20% TBE acrylamide gel (Invitrogen, Carlsbad, Calif., USA) along with ano ligase negative control and a MW marker to confirm Xprobe ligation.

In a second step, the Xpandomer intermediate is treated withRibonuclease H to cleave the RNase labile 5′-3′ phosphodiester bond toproduce an Xpandomer product which is likewise confirmed by gelelectrophoresis.

EXAMPLE 10 Preparation of an Alpha-Phosphate Linker Construct

Tether linkers may also be attached to the phosphorothioate S diester orto a phosphoramidate N-amide, as is discussed by Agrawal (“Site specificfunctionalization of oligonucleotides for attaching two differentreporter groups”, Nuc. Acids Res. 18:5419-23, 1990). Disclosed is amethod for functionalization of two different inter-nucleotide backbonelinkages: aminohexyl phosphoramidate (N-1 amino alkyl) linker andphosphorothioate. The C6 amine prepared as described by Agrawal is usedas a linker for synthesis of an inter-nucleobase tether construct of thepresent invention. Derivatization of N3′-P5′ bonds has also beenreported (Sinyakov et al., “Functionalization of the oligonucleotidescontaining an internucleotide phosphoramidate bond”, Russian JBioorganic Chem, 29:100-102, 2003).

EXAMPLE 11 Heterobifunctional Routes to Tether Constructs

Synthesis of modified oligomers containing a selectively cleavable bondare described in Examples 1-5. Here, a 4mer with a C6-amino modifiedbase in the second position and 4-formylbenzoate-linker modified base inthe third position of the 4mer is prepared by standard oligomersynthetic chemistry. The second and third bases are separated by aselectively cleavable bond selected from ribosyl 5′-3′ phosphodiesterbond, deoxyribosyl 5′-3′ phosphodiester bond, phosphorothiolate bond (5′O—P—S 3′ or 5′ S—P—O 3′), phosphoramidate bond (5′ O—P—N 3′ or 5′ N—P—O3′), or a photocleavable bond. The amine of the C6 linker is reactedwith Sulfo EGS to form an active ester NHS group. Heterobifunctionaltether loop synthesis is then accomplished by reacting the modifiedprobe member with a tether functionalized with amine and hydrazide endterminal groups. A tether-circularized end product is obtained.

EXAMPLE 12 Dye-Labeled Tether Construct

An Xprobe substrate construct with a single reporter is prepared. A 2merwith C6 amines on the first and second bases is first synthesized usingstandard methods. The first and second bases are separated by aselectively cleavable bond selected from ribosyl 5′-3′ phosphodiesterbond, deoxyribosyl 5′-3′ phosphodiester bond, phosphorothiolate bond (5′O—P—S 3′ or 5′ S—P—O 3′), phosphoramidate bond (5′ O—P—N 3′ or 5′ N—P—O3′), or a photocleavable bond. In this example, the tether is aspecies-specific, end-functionalized PEG 2000 molecule with a singleinternal linker group, which in this case is a maleimido-functionalizedlinker located mid-tether. The reporter is a dye-labeled dendrimerattached to the maleimido linker group on the tether via a sulfhydrylmoiety on the dendrimer. The resulting reporter construct therebycontains one reporter on the tether. A cystamine dendrimer (DendriticNanotechnologies, Mt Pleasant, Mich., USA; Cat No DNT-294 G3) with a 5.4nm diameter and 16 surface amines per half-dendrimer) is used as thereporter. By attaching specific dyes, or combinations of dyes to theamine groups on the dendrimer the substrate construct species isuniquely labeled for identification.

EXAMPLE 13 Peptide-Labeled Tether Construct

An Xprobe substrate construct with a single reporter is prepared. A 2merwith C6 amines on the first and second bases is first synthesized usingstandard methods. The first and second bases are separated by aselectively cleavable bond selected from ribosyl 5′-3′ phosphodiesterbond, deoxyribosyl 5′-3′ phosphodiester bond, phosphorothiolate bond (5′O—P—S 3′ or 5′ S—P—O 3′), phosphoramidate bond (5′ O—P—N 3′ or 5′ N—P—O3′), or a photocleavable bond. In this example, the tether is aspecies-specific, end-functionalized PEG 2000 molecule with a singleinternal linker group, which in this case is a maleimido-functionalizedlinker located mid-tether. The reporter is a dye-labeled dendrimerattached to the maleimido linker group on the tether via a sulfhydrylmoiety on the dendrimer. The resulting reporter construct therebycontains one reporter on the tether. A cystamine dendrimer (DendriticNanotechnologies, Mt Pleasant, Mich., USA; Cat No DNT-294 G3) with a 5.4nm diameter and 16 surface amines per half-dendrimer) is used as thereporter. By attaching specific dyes, or combinations of dyes to theamine groups on the dendrimer the substrate construct species isuniquely labeled for identification. By attaching specific peptides tothe dendrimer, which is amine functionalized, the substrate constructspecies is tagged for later identification. The dimensions and thecharge of the attached peptides are used as detection characteristics ina detection apparatus.

EXAMPLE 14 Heterobifunctional Route to Tether Reporter Constructs

Substrate constructs with multiple reporters are prepared. A library of2mers with C6-amino modified base in the first position and4′-formylbenzoate modified base (4FB) in the second position of the 2meris prepared by standard organic chemistry. The first and second basesare separated by a selectively cleavable bond selected from ribosyl5′-3′ phosphodiester bond, deoxyribosyl 5′-3′ phosphodiester bond,phosphorothiolate bond (5′ O—P—S 3′ or 5′ S—P—O 3′), phosphoramidatebond (5′ O—P—N 3′ or 5′ N—P—O 3′), or a photocleavable bond. The amineis reacted with Sulfo EGS to form an active ester NHS group. In a secondstep, a species-specific bifunctional amine reporter segment (segment 1)is reacted with the active NHS group and a species-specific bifunctionalhydrazide reporter segment (segment 4) is reacted with 4FB. Next, thefree amine on segment 1 is reacted with Sulfo EGS to form an activeester NHS. A species-specific heterobifunctional cap consisting of apair of reporter segments (segments 2 and 3) with amine and 4FB endgroups is then reacted with the construct, closing the tether loop.

The resulting reporter construct thereby contains four reporters on thetether construct. In this example, the reporters are polyamine dendronswith a cystamine linker to covalently bind to each polymer segment bythioether bonds (Dendritic Nanotechnologies, Mt Pleasant Mich., USA; CatNo DNT-294: G4, 4.5 nm diameter, 32 surface amines per half-dendrimer)and the polymer members are end-functionalized PEG 2000, each with aninternal linker group. Each tether segment is comprised of a singlereporter. With four directionally coupled segments, 2⁴ possible reportercode combinations are available.

EXAMPLE 15 Heterobifunctional Route to Reporter Constructs withPost-Synthesis Labeling

Substrate constructs with multiple reporters are prepared. A 4mer withC6-amino modified base in the first position and 4′-formylbenzoatemodified base (4FB) in the second position of the 2mer is prepared bystandard organic chemistry. The second and third bases are separated bya selectively cleavable bond selected from ribosyl 5′-3′ phosphodiesterbond, deoxyribosyl 5′-3′ phosphodiester bond, phosphorothiolate bond (5′O—P—S 3′ or 5′ S—P—O 3′), phosphoramidate bond (5′ O—P—N 3′ or 5′ N—P—O3′), or a photocleavable bond. The amine is reacted with Sulfo EGS toform an active ester NHS group. In a second step, a species-specificbifunctional amine reporter segment (segment 1) is reacted with theactive NHS group and a species-specific bifunctional hydrazide reportersegment (segment 4) is reacted with 4FB. Next, the free amine on segment1 is reacted with Sulfo EGS to form an active ester NHS. Aspecies-specific heterobifunctional cap consisting of a pair of reportersegments (segments 2 and 3) with amine and 4FB end groups is thenreacted with the construct, closing the tether loop.

The resulting reporter construct thereby contains four reporters on thetether construct. In this example, post labeling of the tether constructis described. Covalently bound to each tether segment is a 16meroligomer which is used for reporter attachment. Each tether segment iscomposed in part of a functionalized PEG molecule. The reporter is dyelabeled polyamine dendrimer with a cystamine linker (DendriticNanotechnologies, Mt Pleasant Mich., USA; Cat No DNT-294: G5, 4.5 nmdiameter, 64 surface amines per half-dendrimer) for coupling to the16mer oligomer probe. Following assembly of the Xpandomer, thedye-labeled dendrimers are hybridized to the oligomeric tether segments.This labeling approach is analogous to the method described by DeMatteiet al. (“Designed Dendrimer Syntheses by Self-Assembly of Single-Site,ssDNA Functionalized Dendrons”, Nano Letters, 4:771-77, 2004).

EXAMPLE 16 Dye-Labeled Reporter Elements

Referring to Examples 14 and 15, the surface amines of the dendrimericreporter elements are labeled by active ester chemistry. Alexa Fluor 488(green) and Alexa Fluor 680 (red) are available for one-step attachmentas sNHS active esters from Molecular Probes (Eugene Oreg.). The densityand ratio of the dyes are varied to produce a distinctive molecular tagon each reporter element.

EXAMPLE 17 Multi-State Reporter Elements

Various palettes of dyes are selected by techniques similar to thoseused in M-FISH and SKY, as is known to those skilled in the art, andconjugated to a dendrimeric reporter. Thus, the reporter element of eachtether constitutes a “spectral address”, whereby a single dendrimericconstruct with a multiplicity of dye binding sites can create aplurality of reporter codes. Referring to the tether construct describedin Examples 14 and 15, a 5-state spectral address results in 625reporter code combinations.

EXAMPLE 18 PEG-5000 Spacer Tether

Tether segments are constructed of a durable, aqueous/organic solventsoluble polymer that possesses little to no binding affinity for SBXreactants. Modified PEG 5000 is used for the flexible tether spacersthat flank a poly lysine 5000 reporter. The free PEG ends arefunctionalized for attachment to the probe member. The polymer iscircularized by crosslinking to probe attachment points usingheterobifunctional linker chemistry.

EXAMPLE 19 Preparation of a Mass Tag Reporter Composition

A tether is synthesized as follows: Cleavable mass tags are covalentlycoupled to poly-1-lysine peptide functionalized G5 Dendrimers (DendriticNanotechnologies, Mt Pleasant Mich.) using standard amine couplingchemistry. The G5 Dendrimers are ˜5.7 nm in diameter and provide 128reactive surface groups. A string of ten G5 Dendrimers functionalizedwith 10,000 Molecular Weight polylysine peptides provides about 100,000reporter attachment sites collectively in a ˜57 nm dendrimer segment. Atotal of about 10,000 mass tags are available for detection on the fullyassembled segment, assuming only 10% occupancy of the available bindingsites. Using the 3 mass tag reporter coding method described previously(FIG. 37), about 3,300 copies of each mass tag is available formeasurement. As an alternative, a single G9 Dendrimer (with 2048reactive groups) functionalized with 10,000 Molecular Weight poly-lysinehas available about 170,000 mass tag attachment sites in a 12 nm segmentof the reporter construct.

To detect a sequence with mass tags, a method for controlled release ofthe mass tag reporters at the point of measurement by use ofphotocleavable linkers is used. Sequential fragmentation of the tetheris not necessary. Mass tag reporters associated with each subunit of theXpandomer polymer are measured in one step. For example, a set of 13mass tag reporters that range from 350 Daltons to 710 Daltons (i.e., a30 Dalton ladder of mass tag reporters) has 286 combinations of threemass tags each. In this manner, any one of 256 different 4mers isassociated with only one particular combination of 3 mass tag reporters.The encoded sequence information of the Xpandomer is readily detected bymass spectroscopy of the subunits. Because the subunits of the Xpandomerare spatially well separated, the Xpandomer handling and detectiontechnology need not be highly sophisticated.

EXAMPLE 20 Direct Analysis of Unlabelled Substrate Constructs

A substrate construct library is synthesized; the tethers contain noreporters. Following preparation of an Xpandomer product, the individualbases of the Xpandomer product are analyzed by electron tunnelingspectroscopy.

EXAMPLE 21 Hybridization-Assisted Analysis of Unlabeled SubstrateConstructs

An oligomeric substrate construct library is synthesized; the tetherscontain no reporters. Following preparation of an Xpandomer product, acomplete set of labeled probes are then hybridized to the Xpandomerproduct and the duplexed probes are analyzed sequentially.

EXAMPLE 22 Synthesis of a DeoxyAdenosine Triphosphate withLys-Cys-Peg-Polyglutamate-Peg-Cys-COOH Tether

Lysine with a BOC-protected amino side chain is immobilized on a resinand reacted with a cysteine residue using standard peptide synthesismethods. The amino sidechain of the lysine will be the epsilon reactivefunctional group of the RT-NTP tether (see Class VI, VII). The cysteinewill form a first half of an intra-tether disulfide bond. Thedeprotected amine on the cysteine is modified with SANH (Pierce-ThermoFisher, USA; Cat No 22400: Bioconjugate Toolkit) to form a hydrazide.

Separately, a spacer segment is prepared from bis-amino PEG 2000(Creative PEGWorks, Winston Salem N.C.; Cat No PSB 330) byfunctionalization of the free amines with C6-SFB (Pierce-Thermo Fisher,USA; Cat No 22400: Bioconjugate Toolkit), forming a bis-4FB PEG spacersegment; the product is purified.

The bis-4FB PEG spacer segment is then coupled to the hydrazide linkeron the cysteine, leaving a 4FB group as the terminal reactive group, andthe resin is washed.

Separately, a polyglutamate segment (each glutamate derivatized at thegamma-carboxyl with 5 PEO units of methyl-capped PEG) is prepared. Thec-terminus is converted to an amine with EDC coupling agent and diaminohexane. SANH is used to form a di-hydrazide terminated polyglutamatesegment, and the product is purified. The dihydrazide-terminatedpolyglutamate segment is reacted with the terminal 4FB group on theresin, forming a terminal hydrazide, and the resin is washed.

Separately, a PEG-2000 spacer segment (with amine and carboxyl ends,Creative PEGWorks; PHB-930) is reacted with SFB to generate a 4FBterminal group. This spacer segment is reacted with the hydrazide groupon the resin, forming a carboxyl-terminated chain. The resin is againwashed.

A cysteine residue is coupled to the free carboxyl using standardpeptide synthesis reagents. The terminal carboxyl of the cysteine isO-Benzyl protected. The resulting product is again washed and thencleaved from the resin. The free carboxyl generated by cleavage is thenmodified with EDC, aminohexyl, and SANH to form a reactive hydrazide.

Separately, a C6 amine modified deoxyAdenosine triphosphate(N-6-(6-amino)hexyl-dATP, Jena Bioscience, Jena DE; Cat No NU-835) istreated with SFB (Pierce Bioconjugate Toolkit, Cat No 22419) to form a4FB functional group. By combining the modified base with the reactivehydrazide of the preceding steps, a tether-probe substrate construct isassembled. The BOC of the lysine side chain is removed before use. Undergenerally oxidizing conditions, the cysteines associate to form anintra-tether disulfide bond, stabilizing the tether in a constrained,compact form.

EXAMPLE 23 Synthesis of a RT-NTP Triphosphate Library

Tethered nucleotide triphosphate bases A, T, C and G with intra-tether—S—S— bond are prepared as described in Example 22, but charge andphysical parameters of the PEGylated glutamate segments used for eachbase are selected to provide a distinct reporter characteristic.

EXAMPLE 24 Xpandomer Synthesis by SBE Using Polyglutamate TetheredRT-NTPs

Modified RT-NTP Adenosine and Guanosine nucleotide triphosphates withdisulfide intra-tether bonds are prepared. The bases are furthermodified so as to be reversibly blocked at the 3′-position. Allyl-basedreversible blocking chemistry is as described by Ruparel (“Design andsynthesis of a 3′-O-allyl photocleavable fluorescent nucleotide as areversible terminator for DNA sequencing by synthesis” PNAS,102:5932-37, 2005). The tethers of the modified bases are constructedwith delta functional group and epsilon functional group generally asshown in FIG. 61. The delta functional group is a carboxyl of a pendantcysteine of the tether and the epsilon functional group is a side chainamine of a lysine near the attachment of the tether to the purines. Thetethers are further modified so as to contain nucleobase-specificmodified polyglutamate segments.

The sequence TCTCTCTCTCTCTCTCATCTACCGTCCGTCCC” is used as a template.The sequence “5′GGGACGGACGGTAGAT” is used as a primer. A 5′ terminal HEX(5′hexachloro-fluorescein) on the primer is used as a label. The methodof Xpandomer synthesis is essentially as described for FIG. 61. In afirst priming cycle of SBE, the modified nucleobase is added with Klenowpolymerase under conditions adapted for polymerization and a single baseis added to the nascent daughter strand on the 3′-OH end of the primer.Because the substrate construct is blocked at the 3′ position, nofurther polymerization occurs.

The amino side chain (epsilon) linker group on the first RT-NTP added iscapped and will remain so throughout the SBE reaction. The terminalcarboxyl group of the tether is deprotected and the 3′ OH on thesubstrate is unblocked; the complex is washed before a next round ofSBE.

In a second cycle of SBE, another nucleobase is polymerized to thenascent Xpandomer Intermediate. The X bond is formed between the freeamine of the epsilon linker group on the first nucleobase and thecarboxyl linker group on the tether of the second nucleobase using EDCand Sulfo-NHS as a crosslinking agent (Pierce Cat No 22980 and 24510).The carboxyl on the delta linker group of the tether is deprotected andthe 3′ OH on the substrate is unblocked; the complex washed before anext round of SBE.

The cycle of SBE can be repeated multiple times, thereby forming anXpandomer intermediate in the constrained configuration. Each tether inthe growing chain of χ-bonded nucleobases is in the constrainedXpandomer configuration.

EXAMPLE 25 Nuclease and TCEP Cleavage To Form Class X Xpandomer

The Xpandomer intermediate of Example 24 is cleaved with nuclease,forming an Xpandomer product composed of individual nucleobases linkedby tether segments and χ bonds. The nuclease also degrades the templateand any associated primer, freeing the product. The intra-tetherdisulfide linkages are cleaved by the addition of a reducing agent(TCEP, Pierce Cat. No. 20490).

The Xpandomer product is filtered and purified to remove truncatedsynthons and nuclease digestion side products. Detection and analysis ofthe linearized Xpandomer can be done using a wide variety of existingand next generation methods.

EXAMPLE 26 Synthesis of a DeoxyAdenosine Triphosphate with anIntratether Photocleavable Linker

Glycine is immobilized on a resin and reacted with a cysteine. Thecysteine amino group is then deprotected and reacted with a glutamate,the glutamate with a sidechain modified with a photolabile linkerterminating in a OBenzyl protected carboxyl, such as a2-nitroveratrylamine linker adapted from that described by Holmes et al.(“Reagents for combinatorial organic synthesis: development of a newO-nitrobenzyl photolabile linker for solid phase synthesis”, J Org Chem,60:2318-19, 1995). The cysteine will be the “epsilon functional group”of the RT-NTP tether. The deprotected amine on the glutamate is modifiedwith SANH (Pierce-Thermo Fisher, USA; Cat No 22400: BioconjugateToolkit) to form a hydrazide. The sidechain of the glutamate will form aphotocleavable intra-tether linker following synthesis of the tether.

Separately, a spacer segment is prepared from bis-amino PEG 2000(Creative PEGWorks, Winston Salem, N.C., USA; Cat No PSB 330) byfunctionalization of the free amines with C6-SFB (Pierce BioconjugateToolkit, Cat No 22423), forming a bis-4FB PEG spacer segment, and theproduct is purified. The bis-4FB PEG spacer segment is then coupled tothe hydrazide linker on the glutamate, leaving a 4FB group as theterminal reactive group, and the resin is washed.

Separately, a polyglutamate segment (with t-butyl protected sidechains)is prepared. The C-terminus is converted to an amine with EDC couplingagent and diamino hexane. SANH is used to form a di-hydrazide terminatedpolyglutamate segment, and the product is purified. Thedihydrazide-terminated polyglutamate segment is reacted with theterminal 4FB group on the resin, forming a terminal hydrazide on theresin, and the resin is washed.

A PEG-2000 spacer segment (with free amino and FMOC-protected aminoends; Cat No PHB-0982, Creative PEGWorks) is modified with C6 SFB toform a 4FB and FMOC-amino modified PEG spacer segment. The 4FB end isreacted with the hydrazide group on the resin, forming an FMOCamino-terminated chain. The resin is again washed.

A lysine residue is coupled to the free amine of the spacer by a peptidebond. The lysine residue is protected on the sidechain by BOC and thealpha-amine of the lysine is protected by FMOC. The OBenzyl terminalcarboxyl of the photocleavable linker and the BOC-protected sidechain ofthe lysine are then deprotected and crosslinked with EDC/Sulfo-NHS tocircularize the tether.

The resulting product is again washed and then cleaved from the resin.The free glycine carboxyl generated by cleavage is then modified withEDC, aminohexyl, and SANH to form a reactive hydrazide.

Separately, a C6 amine modified deoxyAdenosine triphosphate(N-6-(6-amino)hexyl-dATP, Jena Bioscience, Jena DE; Cat No NU-835) istreated with SFB (Pierce Bioconjugate Toolkit, Cat No 22419) to form a4FB functional group. By combining the modified base with the reactivehydrazide of the preceding steps, a tether-probe substrate construct isassembled. The photocleavable intra-tether linker stabilizes the tetherin a constrained, compact form. The FMOC is then removed and the freeterminal amine is reacted with Sulfo-EMCS (Pierce; Cat No 22307) tointroduce a terminal maleimido functional group.

EXAMPLE 27 Xpandomer Synthesis by SBE Using Photocleavable RT-NTPs

Modified RT-NTP Adenosine and Guanosine nucleotide triphosphates withphotocleavable intra-tether bonds are prepared. The bases are furthermodified so as to be reversibly blocked at the 3′-position. Allyl-basedreversible blocking chemistry is as described by Ruparel (“Design andsynthesis of a 3′-O-allyl photocleavable fluorescent nucleotide as areversible terminator for DNA sequencing by synthesis”, PNAS,102:5932-37, 2005). The tethers of the modified bases are constructedwith delta functional group and epsilon functional group generally asshown in FIG. 61. The delta linker group is an amine of a terminalpendant lysine of the tether and the epsilon linker group is asulfhydryl of a cysteine near the point of attachment of the tether. Thetethers are further modified so as to contain species-specific modifiedpolyglutamate segments.

The sequence TCTCTCTCTCTCTCTCATCTACCGTCCGTCCC” is used as a template.The sequence “5′GGGACGGACGGTAGAT” is used as a primer. A 5′ terminal HEX(5′hexachloro-fluorescein) on the primer is used as a label. The methodof Xpandomer synthesis is essentially as described for FIG. 61. In afirst priming cycle of SBE, the modified nucleobase (A) is added withKlenow polymerase under conditions adapted for polymerization and asingle base is added to the nascent daughter strand on the 3′-OH end ofthe primer. Because the substrate construct is blocked at the 3′position, no further polymerization occurs. The immobilizedprimer-template complex is then washed to remove unreacted substrate.

The sulfhydryl side chain (epsilon) linker group on the first RT-NTPadded is capped and will remain so throughout the SBE reaction. Theterminal amino group of the tether is deprotected and the 3′OH on thesubstrate is unblocked; the complex is washed before the next round ofSBE.

In a second cycle of SBE, another nucleobase (G) is polymerized to thenascent Xpandomer intermediate. The χ-bond is formed between the amino(delta linker group) on the first nucleobase and the sulfhydryl (epsilonlinker group) on the tether of the second base using crosslinkingreagent GMBS (Pierce; Cat No 22309). The delta amino linker group on thesecond RT-NTP is deprotected and the 3′ OH of the substrate isunblocked; the complex is washed before the next round of SBE.

The cycle of SBE can be repeated multiple times, thereby forming anXpandomer intermediate in the constrained configuration. Each tether inthe growing chain of χ-bonded nucleobases is in the constrainedXpandomer configuration.

EXAMPLE 28 Nuclease and Photo Cleavage To Form Class X Xpandomer

The Xpandomer intermediate of Example 27 is cleaved with nuclease,forming an Xpandomer product composed of individual nucleobases linkedby tether segments and χ bonds. The nuclease also degrades the templateand any associated primer, freeing the product. The intra-tetherphotocleavable linkages are cleaved by exposure to a UV light.

The Xpandomer product is filtered and purified to remove truncatedsynthons and nuclease digestion side products. Detection and analysis ofthe linearized Xpandomer can be done using a wide variety of existingand next generation methods.

EXAMPLE 29 Synthesis of an RT-NTP Tether In Situ

Using methods of standard peptide synthesis on a solid support, apeptide is prepared having the structure(Resin-C′)-Glu-Cys-(Gly-Ala)₁₀-Pro-Ser-Gly-Ser-Pro-(Ala-Gly)₁₀-Cys-Lys.The terminal amine is reacted with SANH (Pierce, Cat No 22400) to createa hydrazide linker.

Separately, a C6 amine modified deoxyAdenosine triphosphate(N-6-(6-amino)hexyl-dATP, Jena Bioscience, Jena DE; Cat No NU-835) istreated with SFB (Pierce Bioconjugate Toolkit, Cat No 22419) to form a4FB functional group. By combining the modified base with the reactivehydrazide of the preceding steps, a tether-probe substrate construct isassembled.

The construct is then cleaved from the resin. Following deprotection andunder generally oxidizing conditions, the cysteines associate to form anintra-tether disulfide bond, stabilizing the beta hairpin, whichcontains a terminal free carboxyl (a pendant delta linker group on thetether) and a lysine near the point of tether attachment (the sidechainamine an epsilon linker group).

The disulfide is representative of the intra-tether stabilizationdepicted in Classes II, III, VI, VII, and VIII substrate constructs (seeFIGS. 8 and 9), although illustrated here with more specific referenceto monomeric substrate constructs of Classes VI, VII and VIII. Thelength of the unfolded tether, assuming a residue C—C peptide bondlength of 3.8 Å, is about 10 nm, but assumes a compact shape due tohydrogen bonding in the beta-hairpin.

As described by Gellman (“Foldamers, a manifesto”, Acc Chem Res31:173-80, 1998), a broad variety of polymers, not merely peptides, canbe folded into compact shapes. Such polymers include oligopyridines,polyisocyanides, polyisocyanates, poly(triarylmethyl)methacrylates,polyaldehydes, polyproline, RNA, oligopyrrolinones, and oligoureas, allof which have exhibited the capacity to fold into compact secondarystructures and expand under suitable conditions. Therefore, the peptideexamples presented here are representative of a much larger class oftether chemistries, where constraints on the unexpanded tether caninclude hydrogen bonding and hydrophobic interactions, for example, aswell as intra-tether crosslinks.

EXAMPLE 30 Xpandomer Synthesis Using Xprobes

In one SBX embodiment, an Xprobe library of 256 4mer Xprobes ispresented to a surface tethered and elongated single strand DNA targetfor hybridization. The hybridization step continues under a precisethermal cycling routine for promoting long Xprobe chains. Weakly-bound,non-specific probe-target duplexes are removed by a simple wash step,again under precise thermal control. Enzymatic ligation is performed tolink any Xprobe chains along the target DNA followed by a second wash.By repeating the hybridization/wash/ligation/wash cycle, longer ligatedsequences grow at multiple loci along the target DNA until replicationof target template is mostly complete.

Unfilled gaps along the target DNA are filled using a well establishedDNA polymerase and ligase based gap filling process (Lee, “Ligase ChainReaction”, Biologicals, 24(3): 197-199, 1996). Nucleotides incorporatedinto the gaps have a unique reporter code to indicate a gap nucleotide.The completed Xpandomer intermediate, which is composed of the originalDNA target with complementarily duplexed and ligated Xprobes withoccasional 1, 2 or 3 nucleotide gap fillers, is cleaved to produce anXpandomer. The cleavable linker for this example is a 3′ O—P—N 5′substrate backbone modification. Selective cleavage is catalyzed by theaddition of acetic acid at room temperature.

The Xpandomer is filtered and purified to remove truncated products andis subsequently elongated to form a linear structure of linked reportercodes. Detection and analysis of the Xpandomer product can be done usinga wide variety of existing methods.

EXAMPLE 31 XNTP Xpandomer Synthesis Using Polymerase

Synthesis of a Class X Xpandomer is performed using a modified8-[(6-amino)hexyl]-amino-deoxyAdenosine triphosphate substrate constructhaving a mixed backbone consisting of a non-bridging 2-aminoethylphosphonate and a bridging phosphorothiolate (3′ O—P—S 5′) at the alphaphosphate. An intranucleotide tether is attached to the 2-aminoethylphosphonate linker and to a C6 amino linker on the8-[(6-amino)hexyl]-amino-deoxyATP. The sequenceTTTTTTTTTTTTTTTTTTTTATCTACCGTCCGTCCC” is used as a template. Thesequence “5′GGGACGGACGGTAGAT” is used as a primer. A 5′ terminal HEX(5′hexachloro-fluorescein) on the primer is used as a label. Annealingof the primer and template forms a template with duplex primer and free3′-OH and 5′ terminal single stranded overhang of twenty bases inlength. The substrate constructs and polymerase are then added andpolymerization is continued for 60 min under conditions adjusted foroptimum polymerization. A sample of the polymerization reaction is mixedwith gel loading buffer and is electrophoresed on a 20% TBE acrylamidegel (Invitrogen, USA) along with a no polymerase negative control and aMW marker to confirm XNTP polymerization.

The Xpandomer intermediate is treated with divalent cation (see Mag etal. 1991. “Synthesis and selective cleavage of an oligodeoxynucleotidecontaining a bridged internucleotide 5′-phosphorothioate linkage”,Nucleic Acids Research, 19(7):1437-1441) to selectively cleave thephosphorothiolate bonds between the tether attachment and thedeoxyribose, which is confirmed by electrophoresis.

EXAMPLE 32 XNTP Xpandomer Synthesis Using Polymerase

Synthesis of a Class X Xpandomer is performed using a modifiedN⁶-(6-amino)hexyl-deoxyAdenosine triphosphate substrate construct havinga mixed backbone consisting of a non-bridging (N-1-aminoalkyl)phosphoramidate and a bridging phosphorothiolate (3′ O—P—S 5′) at thealpha phosphate. An intranucleotide tether is attached to theN-1-aminoalkyl group and to a C6 amino linker on theN⁶-(6-amino)hexyl-deoxyATP. The sequenceTTTTTTTTTTTTTTTTTTTTATCTACCGTCCGTCCC” is used as a template. Thesequence “5′GGGACGGACGGTAGAT” is used as a primer. A 5′ terminal HEX(5′hexachloro-fluorescein) on the primer is used as a label. Annealingof the primer and template forms a template with duplex primer and free3′-OH and 5′ terminal single stranded overhang of twenty bases inlength. The substrate constructs and polymerase are then added andpolymerization is continued for 60 min under conditions adjusted foroptimum polymerization. A sample of the polymerization reaction is mixedwith gel loading buffer and is electrophoresed on a 20% TBE acrylamidegel (Invitrogen, USA) along with a no polymerase negative control and aMW marker to confirm XNTP polymerization.

The Xpandomer intermediate is treated with iodoethanol (see Gish et al(“DNA and RNA sequence determination based on phosphorothioatechemistry”, Science, 240(4858): 1520-1522, 1988) or by cleavage withdivalent metal cations as described by Vyle et al (“Sequence- andstrand-specific cleavage in oligodeoxyribonucleotides and DNA containing3′-thiothymidine”. Biochemistry 31(11): 3012-8, 1992) to selectivelycleave the phosphorothiolate bonds between the tether attachment and thedeoxyribose, which is confirmed by electrophoresis.

EXAMPLE 33 XNTP Xpandomer Synthesis Using Ligase

Synthesis of an Class X Xpandomer is performed using a modified8-[(6-amino)hexyl]-amino-deoxyAdenosine monophosphate substrateconstruct having a mixed backbone consisting of a non-bridging2-aminoethyl phosphonate and a bridging phosphoramidate (3′-O—P—N-5′) atthe alpha phosphate. An intranucleotide tether is attached to the2-aminoethyl linker and to a C6 amino linker on the8-[(6-amino)hexyl]-amino-deoxyAMP. The sequenceTTTTTTTTTTTTTTTTTTTTATCTACCGTCCGTCCC” is used as a template. Thesequence “5′GGGACGGACGGTAGAT” is used as a primer. A 5′ terminal HEX(5′hexachloro-fluorescein) on the primer is used as a label. Annealingof the primer and template forms a template with duplex primer and free3′-OH and 5′ terminal single stranded overhang of twenty bases inlength. The substrate constructs and ligase are then added and ligationis continued for 5 hours under conditions adjusted for ligation. Asample of the ligation reaction is mixed with gel loading buffer and iselectrophoresed on a 20% TBE acrylamide gel (Invitrogen, USA) along witha no polymerase negative control and a MW marker to confirm XNTPligation.

The Xpandomer intermediate is treated with 80% acetic acid for 5 hrs atroom temperature according to the procedure of Mag et al (“Synthesis andselective cleavage of oligodeoxyribonucleotides containing non-chiralinternucleotide phosphoramidate linkages”, Nucl. Acids Res. 17: 5973-88,1989) to selectively cleave the phosphoramidate bonds between the tetherattachment point and the deoxyribose, which is confirmed byelectrophoresis.

The various embodiments described above can be combined to providefurther embodiments. All of the U.S. patents, U.S. patent applicationpublications, U.S. patent applications, foreign patents, foreign patentapplications and non-patent publications referred to in thisspecification and/or listed in the Application Data Sheet, areincorporated herein by reference, in their entirety. Aspects of theembodiments can be modified, if necessary to employ concepts of thevarious patents, applications and publications to provide yet furtherembodiments. These and other changes can be made to the embodiments inlight of the above-detailed description. In general, in the followingclaims, the terms used should not be construed to limit the claims tothe specific embodiments disclosed in the specification and the claims,but should be construed to include all possible embodiments along withthe full scope of equivalents to which such claims are entitled.Accordingly, the claims are not limited by the disclosure.

1. A method for sequencing a target nucleic acid, comprising: a)providing a daughter strand produced by a template-directed synthesis,the daughter strand comprising a plurality of subunits coupled in asequence corresponding to a contiguous nucleotide sequence of all or aportion of the target nucleic acid, wherein the individual subunitscomprise a tether, at least one probe or nucleobase residue, and atleast one selectively cleavable bond; b) cleaving the at least oneselectively cleavable bond to yield an Xpandomer of a length longer thanthe plurality of the subunits of the daughter strand, the Xpandomercomprising the tethers and reporter elements for parsing geneticinformation in a sequence corresponding to the contiguous nucleotidesequence of all or a portion of the target nucleic acid; and c)detecting the reporter elements of the Xpandomer.
 2. The method of claim1, wherein the reporter elements for parsing the genetic information areassociated with the tethers of the Xpandomer.
 3. The method of claim 1,wherein the reporter elements for parsing genetic information areassociated with the daughter strand prior to cleavage of the at leastone selectively cleavable bond.
 4. The method of claim 1, wherein thereporter elements for parsing genetic information are associated withthe Xpandomer after cleavage of the at least one selectively cleavablebond.
 5. The method of claim 1, wherein the Xpandomer further comprisesall or a portion of the at least one probe or nucleobase residue.
 6. Themethod of claim 5, wherein the reporter elements for parsing the geneticinformation are or are associated with the at least one probe ornucleobase residue.
 7. The method of claim 1, wherein the at least oneselectively cleavable bond is a covalent bond.
 8. The method of claim 1,wherein the at least one selectively cleavable bond is an intra-tetherbond.
 9. The method of claim 1, wherein the at least one selectivelycleavable bond is a bond between or within probes or nucleobase residuesof the daughter strand.
 10. The method of claim 1, wherein the at leastone selectively cleavable bond is a bond between the probes ornucleobase residues of the daughter strand and a target template. 11.The method of claim 1, wherein the Xpandomer comprises the followingstructure:

wherein T represents the tether; P¹ represents a first probe moiety; P²represents a second probe moiety; κ represents the κ^(th) subunit in achain of m subunits, where m is an integer greater than three; and αrepresents a species of a subunit motif selected from a library ofsubunit motifs, wherein each of the species comprises sequenceinformation of the contiguous nucleotide sequence of a portion of thetarget nucleic acid.
 12. The method of claim 11, wherein the daughterstrand, prior to cleavage of the at least one selectively cleavablebond, comprises a template-daughter strand duplex having the followingstructure:

wherein T represents the tether; P¹ represents a first probe moiety; P²represents a second probe moiety; ˜ represents the at least oneselectively cleavable bond; P^(1′) represents a contiguous nucleotidesequence of at least one nucleotide residue of the template strand towhich P¹ is complementary; P^(2′) represents a contiguous nucleotidesequence of at least one nucleotide residue of the template strand towhich P² is complementary; κ represents the κ^(th) subunit in a chain ofm subunits, where m is an integer greater than three; and α represents aspecies of a subunit motif selected from a library of subunit motifs,wherein each of the species is complementary to the contiguousnucleotide sequence of a portion of the target nucleic acid.
 13. Themethod of claim 11, wherein the daughter strand is formed from aplurality of oligomer substrate constructs having the followingstructure:

wherein T represents the tether; P¹ represents a first probe moiety; P²represents a second probe moiety; ˜ represents the at least oneselectively cleavable bond; and R¹ and R² represent the same ordifferent end groups for the template directed synthesis of the daughterstrand.
 14. The method of claim 1, wherein the Xpandomer comprises thefollowing structure:

wherein T represents the tether; P¹ represents a first probe moiety; P²represents a second probe moiety; κ represents the κ^(th) subunit in achain of m subunits, where m is an integer greater than three; αrepresents a species of a subunit motif selected from a library ofsubunit motifs, wherein each of the species comprises sequenceinformation of the contiguous nucleotide sequence of a portion of thetarget nucleic acid; and χ represents a bond with the tether of anadjacent subunit.
 15. The method of claim 14, wherein the daughterstrand, prior to cleavage of the at least one selectively cleavablebond, comprises a template-daughter strand duplex having the followingstructure:

wherein T represents the tether; P¹ represents a first probe moiety; P²represents a second probe moiety; ˜ represents the at least oneselectively cleavable bond; P^(1′) represents a contiguous nucleotidesequence of at least one nucleotide residue of the template strand towhich P¹ is complementary; P^(2′) represents a contiguous nucleotidesequence of at least one nucleotide residue of the template strand towhich P² is complementary; κ represents the κ^(th) subunit in a chain ofm subunits, where m is an integer greater than three; α represents aspecies of a subunit motif selected from a library of subunit motifs,wherein each of the species is complementary to the contiguousnucleotide sequence of a portion of the target nucleic acid; and χrepresents a bond with the tether of an adjacent subunit.
 16. The methodof claim 14, wherein the daughter strand is formed from a plurality ofoligomer substrate constructs having the following structure:

wherein T represents the tether; P¹ represents a first probe moiety; P²represents a second probe moiety; R¹ and R² represent the same ordifferent end groups for the template directed synthesis of the daughterstrand; ε represents a first linker group; δ represents a second linkergroup; and “----” represents a cleavable intra-tether crosslink.
 17. Themethod of claim 1, wherein the Xpandomer comprises the followingstructure:

wherein T represents the tether; P¹ represents a first probe moiety; P²represents a second probe moiety; κ represents the κ^(th) subunit in achain of m subunits, where m is an integer greater than three; αrepresents a species of a subunit motif selected from a library ofsubunit motifs, wherein each of the species comprises sequenceinformation of the contiguous nucleotide sequence of a portion of thetarget nucleic acid; and χ represents a bond with the tether of anadjacent subunit.
 18. The method of claim 17, wherein the daughterstrand, prior to cleavage of the at least one selectively cleavablebond, comprises a template-daughter strand duplex having the followingstructure:

wherein T represents the tether; P¹ represents a first probe moiety; P²represents a second probe moiety; ˜ represents the at least oneselectively cleavable bond; P^(1′) represents a contiguous nucleotidesequence of at least one nucleotide residue of the template strand towhich P¹ is complementary; P^(2′) represents a contiguous nucleotidesequence of at least one nucleotide residue of the template strand towhich P² is complementary; κ represents the κ^(th) subunit in a chain ofm subunits, where m is an integer greater than three; α represents aspecies of a subunit motif selected from a library of subunit motifs,wherein each of the species is complementary to the contiguousnucleotide sequence of a portion of the target nucleic acid; and χrepresents a bond with the tether of an adjacent subunit.
 19. The methodof claim 17, wherein the daughter strand is formed from a plurality ofoligomer substrate constructs having the following structure:

wherein T represents the tether; P¹ represents a first probe moiety; P²represents a second probe moiety; R¹ and R² represent the same ordifferent end groups for the template directed synthesis of the daughterstrand; ε represents a first linker group; δ represents a second linkergroup; and “----” represents a cleavable intra-tether crosslink.
 20. Themethod of claim 1, wherein the Xpandomer comprises the followingstructure:

wherein T represents the tether; P¹ represents a first probe moiety; P²represents a second probe moiety; κ represents the κ^(th) subunit in achain of m subunits, where m is an integer greater than three; αrepresents a species of a subunit motif selected from a library ofsubunit motifs, wherein each of the species comprises sequenceinformation of the contiguous nucleotide sequence of a portion of thetarget nucleic acid; and χ represents a bond with the tether of anadjacent subunit.
 21. The method of claim 20, wherein the daughterstrand, prior to cleavage of the at least one selectively cleavablebond, comprises a template-daughter strand duplex having the followingstructure:

wherein T represents the tether; P¹ represents a first probe moiety; P²represents a second probe moiety; ˜ represents the at least oneselectively cleavable bond; P^(1′) represents a contiguous nucleotidesequence of at least one nucleotide residue of the template strand towhich P¹ is complementary; P^(2′) represents a contiguous nucleotidesequence of at least one nucleotide residue of the template strand towhich P² is complementary; κ represents the κ^(th) subunit in a chain ofm subunits, where m is an integer greater than three; α represents aspecies of a subunit motif selected from a library of subunit motifs,wherein each of the species is complementary to the contiguousnucleotide sequence of a portion of the target nucleic acid; and χrepresents a bond with the tether of an adjacent subunit.
 22. The methodof claim 20, wherein the daughter strand is formed from a plurality ofoligomer substrate constructs having the following structure:

wherein T represents the tether; P¹ represents a first probe moiety; P²represents a second probe moiety; ˜ represents the at least oneselectively cleavable bond; R¹ and R² represent the same or differentend groups for the template directed synthesis of the daughter strand; εrepresents a first linker group; and δ represents a second linker group.23. The method of claim 1, wherein the Xpandomer comprises the followingstructure:

wherein T represents the tether; κ represents the κ^(th) subunit in achain of m subunits, where m is an integer greater than three; αrepresents a species of a subunit motif selected from a library ofsubunit motifs, wherein each of the species comprises sequenceinformation of the contiguous nucleotide sequence of a portion of thetarget nucleic acid; and χ represents a bond with the tether of anadjacent subunit.
 24. The method of claim 23, wherein the daughterstrand, prior to cleavage of the at least one selectively cleavablebond, comprises a template-daughter strand duplex having the followingstructure:

wherein T represents the tether; P¹ represents a first probe moiety; P²represents a second probe moiety; ˜ represents the at least oneselectively cleavable bond; P^(1′) represents a contiguous nucleotidesequence of at least one nucleotide residue of the template strand towhich P¹ is complementary; P^(2′) represents a contiguous nucleotidesequence of at least one nucleotide residue of the template strand towhich P² is complementary; κ represents the κ^(th) subunit in a chain ofm subunits, where m is an integer greater than three; α represents aspecies of a subunit motif selected from a library of subunit motifs,wherein each of the species is complementary to the contiguousnucleotide sequence of a portion of the target nucleic acid; and χrepresents a bond with the tether of an adjacent subunit.
 25. The methodof claim 23, wherein the daughter strand is formed from a plurality ofoligomer substrate constructs having the following structure:

wherein T represents the tether; P¹ represents a first probe moiety; P²represents a second probe moiety; ˜ represents the at least oneselectively cleavable bond; R¹ and R² represent the same or differentend groups for the template directed synthesis of the daughter strand; εrepresents a first linker group; and δ represents a second linker group.26. The method of claim 1, wherein the Xpandomer comprises the followingstructure:

wherein T represents the tether; N represents a nucleobase residue; κrepresents the κ^(th) subunit in a chain of m subunits, where m is aninteger greater than ten; α represents a species of a subunit motifselected from a library of subunit motifs, wherein each of the speciescomprises sequence information of the contiguous nucleotide sequence ofa portion of the target nucleic acid; and χ represents a bond with thetether of an adjacent subunit.
 27. The method of claim 26, wherein thedaughter strand, prior to cleavage of the at least one selectivelycleavable bond, comprises a template-daughter strand duplex having thefollowing structure:

wherein T represents the tether; N represents a nucleobase residue; N′represents a nucleotide residue of the template strand to which N iscomplementary; ˜ represents the at least one selectively cleavable bond;κ represents the κ^(th) subunit in a chain of m subunits, where m is aninteger greater than ten; α represents a species of a subunit motifselected from a library of subunit motifs, wherein each of the speciesis complementary to the contiguous nucleotide sequence of a portion ofthe target nucleic acid; and χ represents a bond with the tether of anadjacent subunit.
 28. The method of claim 26, wherein the daughterstrand is formed from a plurality of monomer substrate constructs havingthe following structure:

wherein T represents the tether; N represents a nucleobase residue; R¹and R² represent the same or different end groups for the templatedirected synthesis of the daughter strand; ε represents a first linkergroup; δ represents a second linker group; and “----” represents acleavable intra-tether crosslink.
 29. The method of claim 1, wherein theXpandomer comprises the following structure:

wherein T represents the tether; κ represents the κ^(th) subunit in achain of m subunits, where m is an integer greater than ten; αrepresents a species of a subunit motif selected from a library ofsubunit motifs, wherein each of the species comprises sequenceinformation of the contiguous nucleotide sequence of a portion of thetarget nucleic acid; and χ represents a bond with the tether of anadjacent subunit.
 30. The method of claim 29, wherein the daughterstrand, prior to cleavage of the at least one selectively cleavablebond, comprises a template-daughter strand duplex having the followingstructure:

wherein T represents the tether; N represents a nucleobase residue; N′represents a nucleotide residue of the template strand to which N iscomplementary; ˜ represents the at least one selectively cleavable bond;κ represents the κ^(th) subunit in a chain of m subunits, where m is aninteger greater than ten; α represents a species of a subunit motifselected from a library of subunit motifs, wherein each of the speciesis complementary to the contiguous nucleotide sequence of a portion ofthe target nucleic acid; and χ represents a bond with the tether of anadjacent subunit.
 31. The method of claim 29, wherein the daughterstrand is formed from a plurality of monomer substrate constructs havingthe following structure:

wherein T represents the tether; N represents a nucleobase residue; R¹and R² represent the same or different end groups for the templatedirected synthesis of the daughter strand; ˜ represents the at least oneselectively cleavable bond; ε represents a first linker group; δrepresents a second linker group; and “----” represents a cleavableintra-tether crosslink.
 32. The method of claim 1, wherein the Xpandomercomprises the following structure:

wherein T represents the tether; N represents a nucleobase residue; κrepresents the κ^(th) subunit in a chain of m subunits, where m is aninteger greater than ten; α represents a species of a subunit motifselected from a library of subunit motifs, wherein each of the speciescomprises sequence information of the contiguous nucleotide sequence ofa portion of the target nucleic acid; and χ represents a bond with thetether of an adjacent subunit.
 33. The method of claim 32, wherein thedaughter strand, prior to cleavage of the at least one selectivelycleavable bond, comprises a template-daughter strand duplex having thefollowing structure:

wherein T represents the tether; N represents a nucleobase residue; N′represents a nucleotide residue of the template strand to which N iscomplementary; ˜ represents the at least one selectively cleavable bond;κ represents the κ^(th) subunit in a chain of m subunits, where m is aninteger greater than ten; α represents a species of a subunit motifselected from a library of subunit motifs, wherein each of the speciesis complementary to the contiguous nucleotide sequence of a portion ofthe target nucleic acid; and χ represents a bond with the tether of anadjacent subunit.
 34. The method of claim 32, wherein the daughterstrand is formed from a plurality of monomer substrate constructs havingthe following structure:

wherein T represents the tether; N represents a nucleobase residue; R¹and R² represent the same or different end groups for the templatedirected synthesis of the daughter strand; ε represents a first linkergroup; δ represents a second linker group; and “----” represents acleavable intra-tether crosslink.
 35. The method of claim 1, wherein theXpandomer comprises the following structure:

wherein T represents the tether; N represents a nucleobase residue; κrepresents the κ^(th) subunit in a chain of m subunits, where m is aninteger greater than ten; α represents a species of a subunit motifselected from a library of subunit motifs, wherein each of the speciescomprises sequence information of the contiguous nucleotide sequence ofa portion of the target nucleic acid; χ¹ represents a bond with thetether of an adjacent subunit; and χ² represents an inter-tether bond.36. The method of claim 35, wherein the daughter strand, prior tocleavage of the at least one selectively cleavable bond, is duplexedwith a template strand to yield a duplex daughter strand having thefollowing structure:

wherein T represents the tether; N represents a nucleobase residue; N′represents a nucleotide residue of the template strand to which N iscomplementary; ˜ represents the at least one selectively cleavable bond;κ represents the κ^(th) subunit in a chain of m subunits, where m is aninteger greater than ten; α represents a species of a subunit motifselected from a library of subunit motifs, wherein each of the speciesis complementary to the contiguous nucleotide sequence of a portion ofthe target nucleic acid; χ¹ represents a bond with the tether of anadjacent subunit; and χ² represents an inter-tether bond.
 37. The methodof claim 35, wherein the daughter strand is formed from a plurality ofmonomer substrate constructs having the following structure:

wherein T represents the tether; N represents a nucleobase residue; R¹and R² represent the same or different end groups for the templatedirected synthesis of the daughter strand; ε₁ and ε₂ represent the sameor different first linker groups; δ₁ and δ₂ represent the same ordifferent second linker groups; and “----” represents a cleavableintra-tether crosslink.
 38. The method of claim 1, wherein the Xpandomercomprises the following structure:

wherein T represents the tether; n¹ and n² represents a first portionand a second portion, respectively, of a nucleobase residue; κrepresents the κ^(th) subunit in a chain of m subunits, where m is aninteger greater than ten; and α represents a species of a subunit motifselected from a library of subunit motifs, wherein each of the speciescomprises sequence information of the contiguous nucleotide sequence ofa portion of the target nucleic acid.
 39. The method of claim 38,wherein the daughter strand, prior to cleavage of the at least oneselectively cleavable bond, is duplexed with a template strand to yielda duplex daughter strand having the following structure:

wherein T represents the tether; N represents a nucleobase residue; N′represents a nucleotide residue of the template strand to which N iscomplementary; V represents an internal cleavage site of the nucleobaseresidue; κ represents the κ^(th) subunit in a chain of m subunits, wherem is an integer greater than ten; and α represents a species of asubunit motif selected from a library of subunit motifs, wherein each ofthe species is complementary to the contiguous nucleotide sequence of aportion of the target nucleic acid.
 40. The method of claim 38, whereinthe daughter strand is formed from a plurality of monomer substrateconstructs having the following structure:

wherein T represents the tether; N represents a nucleobase residue; Vrepresents an internal cleavage site of the nucleobase residue; and R¹and R² represent the same or different end groups for the templatedirected synthesis of the daughter strand.
 41. An oligomer substrateconstruct for use in a template directed synthesis for sequencing atarget nucleic acid, comprising a first probe moiety joined to a secondprobe moiety, each of the first and second probe moieties having an endgroup suitable for the template directed synthesis, and a tether havinga first end and a second end with at least the first end of the tetherjoined to at least one of the first and second probe moieties, whereinthe oligomer substrate construct when used in the template directedsynthesis is capable of forming a daughter strand comprising aconstrained Xpandomer, the constrained Xpandomer having a plurality ofsubunits coupled in a sequence corresponding to the contiguousnucleotide sequence of all or a portion of the target nucleic acid,wherein the individual subunits comprise a tether, the first and secondprobe moieties and at least one selectively cleavable bond.
 42. Theoligomer substrate construct of claim 41 having the following structure:

wherein T represents a tether; P¹ represents a first probe moiety; P²represents a second probe moiety; ˜ represents the at least oneselectively cleavable bond; and R¹ and R² represent the same ordifferent end groups for template directed synthesis.
 43. The oligomersubstrate construct of claim 41 having the following structure:

wherein T represents a tether; P¹ represents a first probe moiety; P²represents a second probe moiety; R¹ and R² represent the same ordifferent end groups for template directed synthesis; ε represents afirst linker group; δ represents a second linker group; and “----”represents a cleavable intra-tether crosslink.
 44. The oligomersubstrate construct of claim 41 having the following structure:

wherein T represents a tether; P¹ represents a first probe moiety; P²represents a second probe moiety; R¹ and R² represent the same ordifferent end groups for template directed synthesis; ε represents afirst linker group; δ represents a second linker group; and “----”represents a cleavable intra-tether crosslink.
 45. The oligomersubstrate construct of claim 41 having the following structure:

wherein T represents a tether; P¹ represents a first probe moiety; P²represents a second probe moiety; ˜ represents the at least oneselectively cleavable bond; R¹ and R² represent the same or differentend groups for template directed synthesis; ε represents a first linkergroup; and δ represents a second linker group.
 46. The oligomersubstrate construct of claim 41 having the following structure:

wherein T represents a tether; P¹ represents a first probe moiety; P²represents a second probe moiety; ˜ represents the at least oneselectively cleavable bond; R¹ and R² represent the same or differentend groups for template directed synthesis; ε represents a first linkergroup; and δ represents a second linker group.
 47. An monomer substrateconstruct for use in a template directed synthesis for sequencing atarget nucleic acid, comprising a nucleobase residue with end groupssuitable for the template directed synthesis, and a tether having afirst end and a second end with at least the first end of the tetherjoined to the nucleobase residue, wherein the monomer substrateconstruct when used in the template directed synthesis is capable offorming a daughter strand comprising a constrained Xpandomer, theconstrained Xpandomer having a plurality of subunits coupled in asequence corresponding to the contiguous nucleotide sequence of all or aportion of the target nucleic acid, wherein the individual subunitscomprise a tether, the nucleobase residue and at least one selectivelycleavable bond.
 48. The monomer substrate construct of claim 47 havingthe following structure:

wherein T represents the tether; N represents a nucleobase residue; R¹and R² represent the same or different end groups for the templatedirected synthesis of the daughter strand; ε represents a first linkergroup; δ represents a second linker group; and “----” represents acleavable intra-tether crosslink.
 49. The monomer substrate construct ofclaim 47 having the following structure:

wherein T represents the tether; N represents a nucleobase residue; R¹and R² represent the same or different end groups for the templatedirected synthesis of the daughter strand; ˜ represents the at least oneselectively cleavable bond; ε represents a first linker group; δrepresents a second linker group; and “----” represents a cleavableintra-tether crosslink.
 50. The monomer substrate construct of claim 47having the following structure:

wherein T represents the tether; N represents a nucleobase residue; R¹and R² represent the same or different end groups for the templatedirected synthesis of the daughter strand; ε represents a first linkergroup; δ represents a second linker group; and “----” represents acleavable intra-tether crosslink.
 51. A monomer substrate constructhaving the following structure:

wherein T represents the tether; N represents a nucleobase residue; R¹and R² represent the same or different end groups for the templatedirected synthesis of the daughter strand; ε₁ and ε₂ represent the sameor different first linker groups; δ₁ and δ₂ represent the same ordifferent second linker groups; and “----” represents a cleavableintra-tether crosslink.
 52. The monomer substrate construct of claim 47having the following structure:

wherein T represents the tether; N represents a nucleobase residue; Vrepresents an internal cleavage site of the nucleobase residue; and R¹and R² represent the same or different end groups for the templatedirected synthesis of the daughter strand.
 53. A duplex daughter strandfor use in a template directed synthesis for sequencing a target nucleicacid, comprising a daughter strand duplexed with a template strand, thedaughter strand comprising a constrained Xpandomer and having aplurality of subunits coupled in a sequence corresponding to acontiguous nucleotide sequence of all or a portion of the target nucleicacid, wherein the individual subunits comprise a tether, at least oneprobe or nucleobase residue, and at least one selectively cleavablebond.
 54. The duplex daughter strand of claim 53 having the followingstructure:

wherein T represents the tether; P¹ represents a first probe moiety; P²represents a second probe moiety; ˜ represents the at least oneselectively cleavable bond; P^(1′) represents a contiguous nucleotidesequence of at least one nucleotide residue of the template strand towhich P¹ is complementary; P^(2′) represents a contiguous nucleotidesequence of at least one nucleotide residue of the template strand towhich P² is complementary; κ represents the κ^(th) subunit in a chain ofm subunits, where m is an integer greater than three; and α represents aspecies of a subunit motif selected from a library of subunit motifs,wherein each of the species is complementary to the contiguousnucleotide sequence of a portion of the target nucleic acid.
 55. Theduplex daughter strand of claim 53 having the following structure:

wherein T represents the tether; P¹ represents a first probe moiety; P²represents a second probe moiety; ˜ represents the at least oneselectively cleavable bond; P^(1′) represents a contiguous nucleotidesequence of at least one nucleotide residue of the template strand towhich P¹ is complementary; P^(2′) represents a contiguous nucleotidesequence of at least one nucleotide residue of the template strand towhich P² is complementary; κ represents the κ^(th) subunit in a chain ofm subunits, where m is an integer greater than three; α represents aspecies of a subunit motif selected from a library of subunit motifs,wherein each of the species is complementary to the contiguousnucleotide sequence of a portion of the target nucleic acid; and χrepresents a bond with the tether of an adjacent subunit.
 56. The duplexdaughter strand of claim 53 having the following structure:

wherein T represents the tether; P¹ represents a first probe moiety; P²represents a second probe moiety; ˜ represents the at least oneselectively cleavable bond; P^(1′) represents a contiguous nucleotidesequence of at least one nucleotide residue of the template strand towhich P¹ is complementary; P^(2′) represents a contiguous nucleotidesequence of at least one nucleotide residue of the template strand towhich P² is complementary; κ represents the κ^(th) subunit in a chain ofm subunits, where m is an integer greater than three; α represents aspecies of a subunit motif selected from a library of subunit motifs,wherein each of the species is complementary to the contiguousnucleotide sequence of a portion of the target nucleic acid; and χrepresents a bond with the tether of an adjacent subunit.
 57. The duplexdaughter strand of claim 53 having the following structure:

wherein T represents the tether; P¹ represents a first probe moiety; P²represents a second probe moiety; ˜ represents the at least oneselectively cleavable bond; P^(1′) represents a contiguous nucleotidesequence of at least one nucleotide residue of the template strand towhich P¹ is complementary; P^(2′) represents a contiguous nucleotidesequence of at least one nucleotide residue of the template strand towhich P² is complementary; κ represents the κ^(th) subunit in a chain ofm subunits, where m is an integer greater than three; α represents aspecies of a subunit motif selected from a library of subunit motifs,wherein each of the species is complementary to the contiguousnucleotide sequence of a portion of the target nucleic acid; and χrepresents a bond with the tether of an adjacent subunit.
 58. The duplexdaughter strand of claim 53 having the following structure:

wherein T represents the tether; P¹ represents a first probe moiety; P²represents a second probe moiety; ˜ represents the at least oneselectively cleavable bond; P^(1′) represents a contiguous nucleotidesequence of at least one nucleotide residue of the template strand towhich P¹ is complementary; P^(2′) represents a contiguous nucleotidesequence of at least one nucleotide residue of the template strand towhich P² is complementary; κ represents the κ^(th) subunit in a chain ofm subunits, where m is an integer greater than three; α represents aspecies of a subunit motif selected from a library of subunit motifs,wherein each of the species is complementary to the contiguousnucleotide sequence of a portion of the target nucleic acid; and χrepresents a bond with the tether of an adjacent subunit.
 59. The duplexdaughter strand of claim 53 having the following structure:

wherein T represents the tether; N represents a nucleobase residue; N′represents a nucleotide residue of the template strand to which N iscomplementary; ˜ represents the at least one selectively cleavable bond;κ represents the κ^(th) subunit in a chain of m subunits, where m is aninteger greater than ten; α represents a species of a subunit motifselected from a library of subunit motifs, wherein each of the speciesis complementary to the contiguous nucleotide sequence of a portion ofthe target nucleic acid; and χ represents a bond with the tether of anadjacent subunit.
 60. The duplex daughter strand of claim 53 having thefollowing structure:

wherein T represents the tether; N represents a nucleobase residue; N′represents a nucleotide residue of the template strand to which N iscomplementary; ˜ represents the at least one selectively cleavable bond;κ represents the κ^(th) subunit in a chain of m subunits, where m is aninteger greater than ten; α represents a species of a subunit motifselected from a library of subunit motifs, wherein each of the speciesis complementary to the contiguous nucleotide sequence of a portion ofthe target nucleic acid; and χ represents a bond with the tether of anadjacent subunit.
 61. The duplex daughter strand of claim 53 having thefollowing structure:

wherein T represents the tether; N represents a nucleobase residue; N′represents a nucleotide residue of the template strand to which N iscomplementary; ˜ represents the at least one selectively cleavable bond;κ represents the κ^(th) subunit in a chain of m subunits, where m is aninteger greater than ten; α represents a species of a subunit motifselected from a library of subunit motifs, wherein each of the speciesis complementary to the contiguous nucleotide sequence of a portion ofthe target nucleic acid; and χ represents a bond with the tether of anadjacent subunit.
 62. The duplex daughter strand of claim 53 having thefollowing structure:

wherein T represents the tether; N represents a nucleobase residue; N′represents a nucleotide residue of the template strand to which N iscomplementary; ˜ represents the at least one selectively cleavable bond;κ represents the κ^(th) subunit in a chain of m subunits, where m is aninteger greater than ten; α represents a species of a subunit motifselected from a library of subunit motifs, wherein each of the speciesis complementary to the contiguous nucleotide sequence of a portion ofthe target nucleic acid; χ¹ represents a bond with the tether of anadjacent subunit; and χ² represents an inter-tether bond.
 63. The duplexdaughter strand of claim 53 having the following structure:

wherein T represents the tether; N represents a nucleobase residue; N′represents a nucleotide residue of the template strand to which N iscomplementary; V represents an internal cleavage site of the nucleobaseresidue; κ represents the κ^(th) subunit in a chain of m subunits, wherem is an integer greater than ten; and α represents a species of asubunit motif selected from a library of subunit motifs, wherein each ofthe species is complementary to the contiguous nucleotide sequence of aportion of the target nucleic acid.
 64. An Xpandomer for use in atemplate directed synthesis for sequencing a target nucleic acid,comprising a plurality of tethers and reporter elements for parsinggenetic information in a sequence corresponding to the contiguousnucleotide sequence of all or a portion of the target nucleic acid. 65.The Xpandomer of claim 64 having the following structure:

wherein T represents the tether; P¹ represents a first probe moiety; P²represents a second probe moiety; κ represents the κ^(th) subunit in achain of m subunits, where m is an integer greater than three; and αrepresents a species of a subunit motif selected from a library ofsubunit motifs, wherein each of the species comprises sequenceinformation of the contiguous nucleotide sequence of a portion of thetarget nucleic acid.
 66. The Xpandomer of claim 64 having the followingstructure:

wherein T represents the tether; P¹ represents a first probe moiety; P²represents a second probe moiety; κ represents the κ^(th) subunit in achain of m subunits, where m is an integer greater than three; αrepresents a species of a subunit motif selected from a library ofsubunit motifs, wherein each of the species comprises sequenceinformation of the contiguous nucleotide sequence of a portion of thetarget nucleic acid; and χ represents a bond with the tether of anadjacent subunit.
 67. The Xpandomer of claim 64 having the followingstructure:

wherein T represents the tether; P¹ represents a first probe moiety; P²represents a second probe moiety; κ represents the κ^(th) subunit in achain of m subunits, where m is an integer greater than three; αrepresents a species of a subunit motif selected from a library ofsubunit motifs, wherein each of the species comprises sequenceinformation of the contiguous nucleotide sequence of a portion of thetarget nucleic acid; and χ represents a bond with the tether of anadjacent subunit.
 68. The Xpandomer of claim 64 having the followingstructure:

wherein T represents the tether; P¹ represents a first probe moiety; P²represents a second probe moiety; κ represents the κ^(th) subunit in achain of m subunits, where m is an integer greater than three; αrepresents a species of a subunit motif selected from a library ofsubunit motifs, wherein each of the species comprises sequenceinformation of the contiguous nucleotide sequence of a portion of thetarget nucleic acid; and χ represents a bond with the tether of anadjacent subunit.
 69. The Xpandomer of claim 64 having the followingstructure:

wherein T represents the tether; κ represents the κ^(th) subunit in achain of m subunits, where m is an integer greater than three; αrepresents a species of a subunit motif selected from a library ofsubunit motifs, wherein each of the species comprises sequenceinformation of the contiguous nucleotide sequence of a portion of thetarget nucleic acid; and χ represents a bond with the tether of anadjacent subunit.
 70. The Xpandomer of claim 64 having the followingstructure:

wherein T represents the tether; N represents a nucleobase residue; κrepresents the κ^(th) subunit in a chain of m subunits, where m is aninteger greater than ten; α represents a species of a subunit motifselected from a library of subunit motifs, wherein each of the speciescomprises sequence information of the contiguous nucleotide sequence ofa portion of the target nucleic acid; and χ represents a bond with thetether of an adjacent subunit.
 71. The Xpandomer of claim 64 having thefollowing structure:

wherein T represents the tether; κ represents the κ^(th) subunit in achain of m subunits, where m is an integer greater than ten; αrepresents a species of a subunit motif selected from a library ofsubunit motifs, wherein each of the species comprises sequenceinformation of the contiguous nucleotide sequence of a portion of thetarget nucleic acid; and χ represents a bond with the tether of anadjacent subunit.
 72. The Xpandomer of claim 64 having the followingstructure:

wherein T represents the tether; N represents a nucleobase residue; κrepresents the κ^(th) subunit in a chain of m subunits, where m is aninteger greater than ten; α represents a species of a subunit motifselected from a library of subunit motifs, wherein each of the speciescomprises sequence information of the contiguous nucleotide sequence ofa portion of the target nucleic acid; and χ represents a bond with thetether of an adjacent subunit.
 73. The Xpandomer of claim 64 having thefollowing structure:

wherein T represents the tether; N represents a nucleobase residue; κrepresents the κ^(th) subunit in a chain of m subunits, where m is aninteger greater than ten; α represents a species of a subunit motifselected from a library of subunit motifs, wherein each of the speciescomprises sequence information of the contiguous nucleotide sequence ofa portion of the target nucleic acid; χ¹ represents a bond with thetether of an adjacent subunit; and χ² represents an inter-tether bond.74. The Xpandomer of claim 64 having the following structure:

wherein T represents the tether; n¹ and n² represent a first portion anda second portion, respectively, of a nucleobase residue; κ representsthe κ^(th) subunit in a chain of m subunits, where m is an integergreater than ten; and α represents a species of a subunit motif selectedfrom a library of subunit motifs, wherein each of the species comprisessequence information of the contiguous nucleotide sequence of a portionof the target nucleic acid.